How do I void an invoice that already has an Accounting Seed billing attached to it?
As long as the billing related to the Sales Invoice doesn't have any Cash Receipts applied to it and the invoice was posted in an open period, you should be able to Unpost the Sales Invoice. Doing this will unpost and delete the billing record. You can then make any changes to the Sales Invoice then post it again.
If there is a Credit Memo applied to the billing record associated with the invoice, you will need to unapply the Credit Memo from the billing record. Once the billing has the full amount outstanding, you will be able to unpost and then make any changes you need to the invoice.