update the table

hi guyz ,

i wanna update the table with cancellation code in the table...this is my code..structure contains the updated field but table doesnt...

LOOP AT gt_header INTO gs_header.

IF NOT gs_header-to_number IS INITIAL.

gs_header-canx_code = '99'.

ELSE.

gs_header-canx_code = '00'.

ENDIF.

ENDLOOP.

CHECK NOT gt_header[] IS INITIAL.

UPDATE ztcptab_orderhdr FROM TABLE gt_header.

IF sy-subrc = 0.

COMMIT WORK AND WAIT.

ENDIF.

Thanks