You can access existing Invoices from a Client Account or from the Invoice List (Accounting > Invoices). On the Invoice List, click any Invoice Number to open the corresponding Invoice overlay. The Invoice overlay will look something like this:
In this scenario, the Invoice is locked so you would need to unlock it before it can be edited. At the top of the Actions menu is a gray button that says "Locked". To unlock the Invoice, click Unlock under Advanced Actions. After you confirm the dialog, this will change the Locked button to an Edit button. To edit the Invoice, click Edit. This will open the Invoice overlay:
You can edit any of the elements on this overlay except the Account. When you're finished making changes, click Save Invoice at the bottom of the overlay.