Go to the Purchase Order Details help to be sure you are authorized to create a Modify or Cancel document for the purchase order.
If you are authorized, but the links to create a Modify or Cancel document are not available from the Purchase Order (PO) page, hover your cursor over the word Modify to see a pop-up message explaining why the link is not active. Sometimes, it is because there is a receipt pending finalization and changes cannot be made to the PO while there are receipts pending finalization.