records not modifying

hi folks,

I am trying to modify the records in the internal table. It is not happening.

data: it_pb4000 type standard table of pb4000 with header line.

select * from pb4000 up to 1 rows into table it_pb4000

where pernr = g_app ORDER BY begda DESCENDING.

if sy-subrc = 0.

read table it_pb4000 index 1.

******the value is cleared...

clear it_pb4000-endda.

*****NOT HAPPENING****

modify it_pb4000 transporting endda where

endda = '99991231'.

modify pb4000 from it_pb4000.

can anyone tell me where I am doing wrong?

Thanks

Vinu.