Update

I added a second example of how you can do this at the bottom.

If you want tabs-style navigation & keep dashboard, you could build the navigation.

You have to pay VERY close attention to what things are called and use a unique id where that’s an option. Everything needs to have unique names on the different dashboards. In my example code, there is already an error. I used the dashboard from your question as the first tab. Anytime any button is pressed, it thinks that the download data button is pressed. (e.g., the refresh button, the tabs, and any others among your pages)

Below is a working example of manual tabbing between dashboards.

I created two dashboards saved in objects db1 and db2. db1 is your initial dashboard.

db1 <- dashboardPage(
  dashboardHeader(title = "dash board header"),
  dashboardSidebar(
    sidebarMenu(
      actionButton("refreshBtn", "Refresh", icon = icon("refresh"), onclick = "refreshPage()"), # Refresh button
      menuItem("Data Upload", tabName = "uploadView", icon = icon("file")),
      menuItem("Averages", tabName = "Averages", icon = icon("list"))
    )
  ),
  dashboardBody(
    tabItems(
      tabItem(tabName = "uploadView",
              fluidRow(
                column(4,
                       fileInput("file1", "Choose CSV File 1",
                                 accept = c("text/csv",
                                            "text/comma-separated-values,text/plain",
                                            ".csv")),
                       fileInput("file2", "Choose CSV file 1",
                                 accept = c("text/csv",
                                            "text/comma-separated-values,text/plain",
                                            ".csv")),
                       hr(),
                       downloadButton("downloadData", "Download Data"),
                       checkboxGroupInput("SelectOption", label = "Select Box", 
                                          choices = c("Option 1", "Option 2"))
                )
              )
      )
    )
  )
)
db2 <- dashboardPage(
  dashboardHeader(title = "dash board header 2"),
  dashboardSidebar(
    sidebarMenu(
      tags$div("not much to see here")
    )
  ),
  dashboardBody(
    tags$div("Nothing to see here")
  )
)

Within the UI, I created the navigation bar, buttons for tab control and added the dashboards.

For each dashboard or tab

• there’s a tags$button within tags$nav (the use of tags$nav is so that it inherits styles from your dashboard)
• there’s a tags$div after tags$nav

Note that in the buttons the text highlighted

tags$button(onclick = HTML("gimmeTab('Dashboard 1')"),...

MUST match what’s assigned to id in the tags$div that holds the matching dashboard

tags$div(id = "Dashboard 1",...

Read through the comments in the code for more info and if you have any questions, let me know.

ui <- 
  fluidPage(
    tags$head(     # this removes a left and right margin
      tags$style(HTML(    
        "body > div.container-fluid {
          padding: 0;
          margin: 0;
        }"
      )),          # adds tabs' control
      tags$script(HTML(      
        "function gimmeTab(chosenOne){
          let i;
          let x = document.getElementsByClassName('tabber');
          for(i = 0; i < x.length; i++) {
            x[i].style.display = 'none';
          }
          document.getElementById(chosenOne).style.display = 'block';
        }"
      ))
    ),
    tags$body(       # 2 primary elements: navigation bar and that which it controls
      tags$nav(class = "navbar navbar-static-top", role = "navigation",
               
                    # here a button for each dashboard; must have class = "navBtn" 
                            # must have unique inner text: first is 'Dashboard 1'
               tags$button(onclick = HTML("gimmeTab('Dashboard 1')"), "Dashboard 1", class = "navBtn"),
                            # unique text: 'Dashboard 2'
               tags$button(onclick = "gimmeTab('Dashboard 2')", "Dashboard 2", class = "navBtn")
      ),      
      # for each unique dashboard
            #  a `tags$div` with class 'tabber' 
            # the tags span exactly as shown here
            # id  that matches button inner text EXACTLY
            # content - the dashboard
      tags$div(id = "Dashboard 1", class = "tabber", 
               tags$span(onclick = HTML("this.parentELement.style.display = 'none'")),
               tags$h2("Dashboard 1"), 
               db1
      ),
      tags$div(id = "Dashboard 2", class = "tabber", 
               tags$span(onclick = "this.parentELement.style.display = 'none'"),
               tags$h2("Dashboard 2"), 
               db2
      )
    )
    
  )

server <- function(input, output, session){ }

shinyApp(ui = ui, server = server)

Based on some of your comments, I thought I would give you another example of how you could do this.

Instead of buttons, this uses HREF (which is what navbar uses). This relies heavily on navbar‘s coding under the surface without hijacking the styles you’re using in your dashboard. Please note the comments explaining what elements are doing what and the things you need to consider if or when you use it.

ui2 <- 
  fluidPage(
    tags$head(    
      tags$style( 
               # in this call to tags$style
               # the first chunk removes a left and right margin
               # the second is the highlight color with mouse and selection
                  # the background color and (text) color what I picked (that I thought looked ok)
        HTML(     
          "div.container-fluid {
          padding: 0;
          margin: 0;
        }
        .navbar-default .navbar-nav>.active>a,
        .navbar-default .navbar-nav>.active>a:hover,
        .navbar-default .navbar-nav>.active>a:focus{
          background-color: #9EC6DE;
          color: white;
        }"
        )) 
    ),
    tags$body(       # 2 primary elements: navigation bar and that which it controls
      # navigation bar
      tags$nav(class = "navbar navbar-default navbar-static-top container-fluid", role = "navigation",
               style = htmltools::css(background.color = "#3c8dbc", color = "white"),        # white text matching blue bg
               tags$ul(class = "nav navbar-nav",
                       
                       # this is the settings for the tabs: 1 for each dashboard; the HREF must match the ID in the next section (less the #)
                            # each dashboard needs it's own tags$li(tags$a...)
                            # in href = "#....." must match the id set in tags$div() with the dashboard
                       tags$li(class = 'active',         # set as default tab
                               tags$a(href = "#dashboard-1", `data-toggle` = "tab", style = "color: white;", "Dashboard 1")),
                            # each of the remaining tabs are formatted like this (without the active designation)
                       tags$li(tags$a(href = "#dashboard-2", `data-toggle` = "tab", style = "color: white;", "Dashboard 2"))
               )
      ),  
      # that which the navigation bar controls
      tags$div(class = "container-fluid",
               tags$div(class = "tab-content",        
               # this is where the dashboards are stored 
               # each dashboard needs its' own tags$div
               # the 'active' tab above is the only div here with the class 'active'
               # in id = "....." must match the href (less #) found in the tags$li()
               
                            # set as tab-pane and default tab with the class 'active'
                        tags$div(id = "dashboard-1", class = "tab-pane active", 
                                 db1
                        ),  # remaining NOT active tabs, one for each tags$li() above
                        tags$div(id = "dashboard-2", class = "tab-pane", db2)  
               )
      )
    )
  )

server <- function(input, output, session){ }

shinyApp(ui = ui2, server = server)