Hi Niru,
As per my understanding, you are creating a custom ALV report for updating BSEG entries. The update process done using FM.
To read BKPF, BSEG we can use either FM like READ_BKPF & READ_BSEG or use SELECT statement for fetching the value from Table and displaying to ALV.
For displaying entries in editable mode, change the field catalog attribute.
Please find the document below.
ALV-Editing and saving the edited values in Database(OOPS) - Code Gallery - SCN Wiki
Editable ALV through OOPS - ABAP Development - SCN Wiki
Regards
Rajkumar Narasimman