I am writing some requests that I need to convert json string as input to some type of Codable as payload. however, I have the error shows cannot assign to property: 'payload' is a 'let' under the function. why is it?

Error:

Line 7: Char 13: error: cannot assign to property: 'payload' is a 'let' constant in solution.swift
    payload.first = "second first"
    ~~~~~~~ ^
Line 8: Char 13: error: cannot assign to property: 'payload' is a 'let' constant in solution.swift
    payload.last = "second last"
    ~~~~~~~ ^

Code:

struct Name: Codable {
    var first: String
    var last: String
}
 
func session(payload: Name) {
    payload.first = "second first"
    payload.last = "second last"
    print(payload)
}


let json = "{"first": "i am first", "last": "i am last"}"
let data = try json.data(using: .utf8)!
var payload = try JSONDecoder().decode(Name.self, from: data)
session(payload: payload)