If you've marked your invoice as paid on Bonsai, it's easy to undo.

Just view your invoice and select "Undo Mark Paid" from the dropdown options.  This can be helpful if some mistake happened, or a check payment outside of Bonsai bounced.

Doing this will revert your invoice to "Outstanding"/"Overdue", as well as change the status of any expenses to "Billed" (instead of "Billed and Paid").

As a result of this, the paid status at the bottom of the invoice (as shown below) will also be removed.
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This paid status did stay intact in the past when a paid invoice was changed to unpaid which caused some confusion.

It will not notify your client, so you are responsible for any communication needed once you've resolved your payment.