Hi Dan,
instead of using loop for it_skat internal table, use read table statement. both will work but performance wise read table is good.
loop at it_upload into wa_upload.
READ TABLE it_skat INTO wa_skat WITH KEY saknr = wa_upload-saknr.
IF sy-subrc EQ 0.
wa_skat-txt20 = wa_upload-text.
MODIFY skat FROM wa_skat.
ENDIF.
endloop.
Thanks,
Sree