To edit an invoice, tap it in the list of invoices, then tap the "Edit" button in the upper right corner of the screen. Make the necessary changes and tap "Save."
Please note that you can only edit unpaid invoices.
To edit a paid invoice, mark it as "Unpaid" first. Your client won't see the changes automatically — you'll need to resend the invoice if you want them to know about the updates. Afterward, mark the invoice as "Paid" again if you want it to appear in your list of paid invoices.