If change management is active, you can only change a purchase order when its status is Draft.
To change a confirmed purchase order, request a change. This action sets the status to Draft.
Steps
Go to Accounts payable > Purchase orders > All purchase orders.
Note
You can also access All purchase orders via the Procurement and sourcing module.
In the list, select the desired record.
Click Request change.
Close the page.
Go to Default.