In my repo, I have one branch (branch_1) and from it I have been committing some changes. Then I created a new branch from branch_1, branch_2.

Master---
         
          Branch_1--commit1--commit2
                                   
                                    Branch_2 (current branch) 

Meanwhile, a review was done in Branch_1 and I did some changes:

  Master---
             
             Branch1--squashed commit1,commit2--commit3--commit4
                                       
                                        Branch2 (current branch)

How can I now apply the last changes of branch_1 in branch_2?
What it means to merge branch_1 onto branch_2? and viceversa
Any explanation on this will be much appreciate it