Hello Swa G,
Regarding Editing of the PO can be done by the Purchaser , so are you having the role for Purchaser just check with your roles. Now as you said its in awaiting approval please check with WS ID in SWI1 what's the status also check under BBP_PD what's the status for your workflow , as it might happen that the PO is approved but your status is not getting updated.. So just for a check.Firstly check with your roles.
Thanks
Gaurav Gautam