This macro uses an inputbox to ask what month you are updating. Use numeric values instead of typing in the month. Example: Jan. =1, Feb. = 2, Mar. = 3. I am assuming you would not want to update January so the macro will exit if 1 is typed in the inputbox (unless it is updated from December then we can change it).
Hope this helps.

Sub Update_Month()

Dim answer As Variant
Dim j
j = 3

answer = InputBox("What month are you updating?" & vbCrLf & _
"Ex: January =1, February = 2, March=3, etc.")


Select Case answer

Case 1

Exit Sub

Case 2

For j = 3 To 6

Range("B" & j) = Range("A" & j)

Next j

Case 3

For j = 3 To 6

Range("C" & j) = Range("B" & j)

Next j

Case 4

For j = 3 To 6

Range("D" & j) = Range("C" & j)

Next j

Case 5

For j = 3 To 6

Range("E" & j) = Range("D" & j)

Next j

Case 6

For j = 3 To 6

Range("F" & j) = Range("E" & j)

Next j

Case 7

For j = 3 To 6

Range("G" & j) = Range("F" & j)

Next j

Case 8

For j = 3 To 6

Range("H" & j) = Range("G" & j)

Next j

Case 9

For j = 3 To 6

Range("I" & j) = Range("H" & j)

Next j

Case 10

For j = 3 To 6

Range("J" & j) = Range("I" & j)

Next j

Case 11

For j = 3 To 6

Range("K" & j) = Range("J" & j)

Next j

Case 12

For j = 3 To 6

Range("L" & j) = Range("K" & j)

Next j

End Select

End Sub