You can edit invoices by clicking the Billing -> Browse Invoices -link it the top menu of the software.

The Billing view shows a list of all invoices that have been created. You can edit or view the invoice you want by clicking the Edit -button on the right side of the desired invoice row or clicking on the invoice number.

An invoice in Invoiced status can be edited until after you clicking the Edit -button at the bottom of the invoice page.