If an invoice is entered and posted, or marked as paid, you can still edit the amount of the invoice.
To update, find the entry either through Search or through the Register of the account.
Click on the Associated Invoice to open the invoice transaction.
From there, you can make updates to the Payment Method, Amount, Dates or Chart of Accounts.
This is relative to Role Permissions and Accounting Period Status.
Important: Changing the Payment Method will move the invoice back from the original bank or credit card account to the Payments page. The invoice will no longer be marked as paid and will require updating from the Payments tab.