To edit an invoice, choose it in the list of invoices by tapping it. Then, the Edit button in the upper right corner of the screen. Make the necessary changes and tap the Save button after that.

Please note that you can only edit the unpaid invoices. To edit a paid invoice, you need to mark it as Unpaid first. Your client won't see the changes automatically, so you need to resend it if you want them to be aware of them. You'll need to mark the invoice as Paid again if you want to see it in the list with paid invoices.