Hi Hasan,
I can see that with reference to zmmtwinfd-wtno you fetching some records in internal and in update statement you never used those records...what i understand from your question you need to copy value of TRFNO to RFMBELNR field of all records where the field value rfwtno = WTNO. if my undrestanding is correct try using fallowing code..
if you have wtno from selection screen then
Select single tfrno from ztable into g_tfrno where wtno = s_wtno.
update ztable set rfmbelnr = trfno where rfwtno = wtno.
If you want all wtno in ztable then.
Select wtno tfrno from ztable into table it_ztable.
loop at it_ztable into wa_ztable.
update ztable set rfmbelnr = wa_ztable-trfno where rfwtno = wa_ztable-wtno.
Endloop.
Regards,
Shashikanth