=HYPERLINK('C:Reports2018[2018.xlsx]Sheet1'!U21)

Dim wkbYear


Private Sub Worksheet_Change(ByVal Target As Range)

    If Target.Address = "$E$1" Then
        ' here i replace the formula 2018 -> 2019
        ' file 1
        Sheets("Sheet2").Range("B6").Formula = Replace(Sheets("Sheet2").Range("B6").Formula, wkbYear, Target)
        ' file 2
        Sheets("Sheet2").Range("D6").Formula = Replace(Sheets("Sheet2").Range("D6").Formula, wkbYear, Target)
        ' I excluded the 20 other similar lines of code here.
    End If

End Sub

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    If Target.Address = "$E$1" Then
        wkbYear = Range("E1").Value ' Old year value
    End If
End Sub