To close an active invoice that you see in your current folder:
Click into the invoice that you'd like to close.
Click on the gold action/arrow button towards the top right of your screen.
Click on the Close Invoice button and reconfirm that you'd like to close this invoice.
Important things to note:
Closing an invoice doesn't delete or remove any payments previously made on this invoice or affect any payments or transactions from your payment reports.
Closing an invoice will move the invoice our of the Current Tab and into the "All" tab, with a status of Closed.
Closing an invoice will treat the unpaid balance as forgiven.