Edit Invoice
If payments have been applied to the invoice, you will need to void the payments first before editing. When viewing an invoice click the Void Payment link for any payments on the invoice in the Payment Detail section. You can also go to Income -> Payments -> Void Payments.
If it's not feasible to void payments, then the next best thing to do is to make a new invoice that contains the changes that are needed. The new invoice combined with the existing invoice will together make the needed target balance.
If you have not applied a payment, you will be able to make changes to the invoice directly.
If the Edit Invoices on POS add-on is active, you can make changes to the invoice and click Save when done. Locate your invoice in Income -> Invoices. Click the Edit button when viewing the invoice info to make changes.
If editing invoices outside the POS tool, you'll need to click the Unpost button first, make changes, Save, then click Post.