Couple of ways
1. WS formula
=RIGHT(K2,9)&CHAR(MONTH(NOW())+64)
2. User Defined Function use multiple times on WS ..., FormatInvoice
3. Macro to do all of Col K
Option Explicit
'Invoice 123456789
'(For August, letter "H" is concatenated, for September it is "I", and so on).
'After macro: 123456789H
Function FormatInvoice(s As String) As String
FormatInvoice = Right(s, 9) & Chr(Month(Now) + 64)
End Function
Sub FormatAllInvoices()
Dim r As Range, r1 As Range
With ActiveSheet
Set r = Range(.Cells(2, 11), .Cells(2, 11).End(xlDown))
End With
For Each r1 In r.Cells
r1.Offset(0, 1).Value = FormatInvoice(r1.Value)
Next
End Sub