Moving Back And Forth Between Files

I am developing a macro where I need to open a file, copy a value from a cell in that file and paste it in a cell of the original file. I need to do this several times from the same two files. My problem is that I don't have a command that moves back to the newly opened file to copy another value and paste it, again, in a cell of the original file. I can't use a specific file name to move to the newly opened file because the new file name changes every day. Here is the command that I use to open a new file:

Dim fn As Variant
fn = Application. GetOpenFilename("Excel-files,*.xls", _
1, "Select The Previous Day's Borrowing File To Open", , False)
If TypeName(fn) = "Boolean" Then Exit Sub
' the user didn't select a file
Debug.Print "Selected file: " & fn
Workbooks.Open fn

I need to move back to file fn several times, but, as I stated above, the file fn changes every day.