Step 1) For changing an existing purchase order you can use transaction code ME22N (or ME22 - the old version).
Immediately after executing transaction code, system will take you to the most recent purchase order you have created, changed or viewed. If you need to change some other purchase order, you can use the option which appears in the menu
Purchase order = > Other Purchase Order.
Step 2)You will be presented a screen to enter the document you want to process.
We can add another item and save our purchase order.