In the Web Portal, go to Accounts > Invoices > select the Edit pen for the invoice. Inside the Invoice, you can select the Status dropdown to change the status.
Alternatively, while in the Invoices list, you can select the checkbox for the invoice that you'd like to edit and then select Change Status at the bottom of the screen. It will bring up a small popup with the status dropdown, so you can select the status and then select the green Change Status button below it.
