method 1
To close a PO, open any Purchase order that you want to close. Click on "Close P.O" button which is near top-right corner as show below
To Open a closed PO, click on More option form the Purchase order which you want to open. And Click on "Open P.O." to Open the Purchase Order.
method 2
To Close multiple Purchase Orders at a time, go to Purchases >> "Batch PO Close"
Select the PO's which you want to close and Click on Close Selected PO's to close them all at once.
You can also use filters to search with special parameters for specific Purchase orders.