After creating an invoice, you are are able to make any changes as long as the invoice is marked as BILLED. (Please see link below for editing a PAID invoice)
To edit an invoice for a member:
Go to the MEMBER screen and perform a FIND for the member whose Invoice you need to edit
Click on the Dues/Rpac tab and click on the BLUE arrow to the left of the invoice.
Once you see the Invoice you need to edit, click on the EDIT button on the top right portion of the invoice. This will being you into the same screen as you normally would see if you were creating a new invoice.
Once you are in the EDIT screen, you can add/edit/delete items and adjust pricing
Once you are done editing, click on the DONE button at top right to save your changes.