Best way to update BSEG-SGTXT

The problem is that BSEG-SGTXT is duplicated in other tables, like BSIS, BSID etc. You need to use a method that performs all the checks and updates that occur when you modify the field manually. If you can't modify the field manually, it would be extremely unwise to update the table directly. The best method would be to find a BAPI which updates accounting documents, and allows you to set this field. Then it is just a matter of calling the BAPI function module. Otherwise, record the manual process using SM35, and create a batch input program.