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 Review Invoice overlay. The overlay will look something like this:
In this scenario, the Invoice is locked (see the gray button that says "Locked" at the top of the Actions menu).
To unlock the Invoice, click Advanced Actions then select Unlock. After you confirm the change, the Locked button will update to an Edit button.
Click Edit to open the Edit Invoice overlay:
You can edit any of the elements on this overlay except the Account.
You can modify any of the existing line items by clicking the field you want to edit.
To add additional services or products to the Invoice, click Add Line. If you're adding or editing a Product line item, be sure to choose your print options:
The totals at the bottom of the Invoice update as you add or edit line items.
Click Add subtotal row to add a Subtotal to the Invoice. Use the Memo field to record internal notes. The Client will not see these. If you need additional text to appear on the Invoice for a Client, you'll have to update the Invoice Description instead.
When you're finished editing, be sure to click Save Invoice at the bottom of the overlay.