Option Explicit

Sub MinusSix()
    Dim i As Long, lr As Long
    lr = Range("A" & Rows.Count).End(xlUp).Row
    Application.ScreenUpdating = False
    For i = 1 To lr    'Assumes data starts in Row 1
        Range("A" & i) = 6 - Range("A" & i)
    Next i
    Application.ScreenUpdating = True

End Sub