I would do this with VBA. The following code will work if you have all your data in Column A.
Code:
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