Hi lalitha,
check out this sample code if you are passing subroutine in I_callback_usercommand of reuse_alv_grid_display.
FORM user_command USING r_ucomm LIKE sy-ucomm
rs_selfield TYPE slis_selfield.
READ TABLE gi_final_output INTO gw_final_output INDEX rs_selfield-tabindex.
modify <database_table> from gw_final_output where <key_filed> = gw_final_output-key_field.
ENDFORM.
Hope it will help.
Regards,
Mohammed Quddus.