Once you create a PO & later on if you require any changes to be done in the PO, then BAPI_PO_CHANGE can be used.
BAPI_PO_CHANGE is used for changing Purchase Orders. It's functionality is same as ME22 transaction. But in ME22 you navigate to each screen and give input. Where as in BAPI you have to fill all the required EXPORT and Table parameters to change PO.
Hope this helps your Understanding.