An invoice can be unmatched from a Purchase order as follows:
go to Invoice Register, Update Invoices.
Find the Invoice and click the blue ellipsis button (three dots) .
If the invoice has been rejected, use the Amend Entry button to edit it and clear the Order Number field. This will enable the Match to Purchase Order option in step 3
Select Match to Purchase Order.
Click Yes to the confirmation message.
Search for the orders.
Unitck the Matched tick box and press Save Match for all relevant orders.
The Invoice will now be unmatched.