From the main admin centre, Head into the suppliers section

Once in the suppliers section, you will find a list of previous purchase orders which have been set up on the right side within the recent orders section.

Or if you wish, you can click into a supplier name from the left side and access any completed purchase orders that way

Once you are in the purchase order, it should show as being completed with the green tab at the top to indicate this

Click the edit button on the right side which will open up an edit screen for the purchase order

Once you have clicked the edit option, you will then be presented with a field where you can use a tick box to mark the order as not completed

Press save to confirm

This will reopen the purchase order for you and mark it as no longer being complete