Hi Kalapana,
Try with the following code.
LOOP AT itab.
IF itab-fkart = 'ZIV'.
SELECT SINGLE name1 INTO name12 FROM kna1 WHERE kunnr = itab-kunag.
if sy-subrc = 0.
itab-name1 = name12.
MODIFY itab TRANSPORTING name1.
endif.
ENDIF.
ENDLOOP.
before modifying the value check sy-subrc otherwise it willl update the null values.
Regards,
Gurunath