What did you do exactly?  What module type of an invoice are you trying to change, AR, SO, JC?  Is the invoice in Data Entry, or in History?  You will need to clarify your question.  

It is in AP and it is a historical invoice.

If you have an invoice on file and payable in Account Payable, you can adjust that invoice to zero through Accounts Payable, Invoice Data Entry.  enter in the Vendor ID and the Invoice Amount, exactly as seen in vendor maintenance, or on an AP Trial Balance.  Press the button to "adjust" when you are warned the invoice is on file.  Key in the amount of the adjustment to bring that invoice to zero; associate the proper GL Account and then post the adjustment.  You can enter the transaction, make a backup copy of your company and post in the backup first, to confirm  you are doing this properly.