Macro open file and go back

You have the Source and Destination mixed up a little.

Sub Update_Feedback()
 '
 
Dim WBSrc As Workbook
 Dim WBDest As Workbook
 Dim sFil As String
 Dim sTitle As String
 Dim sWb As String
 Dim iFilterIndex As Integer
 ' On Error GoTo err_handler
 Set WBDest = ThisWorkbook
 ' Set up list of file filters
 sFil = "Excel Files (*.xl*),*.xl*"
 ' Display *.xls by default
 iFilterIndex = 1
 ' Set the dialog box caption
 sTitle = "Select File to Open"
 ' Get the filename
 sWb = Application.GetOpenFilename(sFil, iFilterIndex, sTitle)

 If sWb <> "False" Then
     Application.ScreenUpdating = False
     Set WBSrc = Workbooks.Open(Filename:=sWb)
     WBSrc.Activate
     Application.ScreenUpdating = True
     WBDest.Sheets("MAIN").Range("D5") = WBSrc.Sheets("Source").Range("B6")
 End If
 Exit Sub
err_handler:
 MsgBox "No selection made"
 
End Sub