In Tally.ERP 9, you can alter or delete a purchase order either from the Day Book or the Inventory Books.
To alter a Purchase Order
1. Go to Gateway of Tally > Display > Day Book.
Or
1. Go to Gateway of Tally > Display > Inventory Books > Purchase Orders Book.
2. Make the necessary alterations.
3. Press Enter to save.
To delete a Purchase Order, press Alt + D in the Purchase Order alteration screen.