Hi,
If I have understood your requirements (which was not easy, because they are not clearly written), you want to loop at table A and update the matching entries in table B, based on the data in table A.
loop at lt_a into ls_a.
loop at lt_b assigning <fs_b>
where key = ls_a-key
[ update <fs_b> based on the values in ls_a ]
endloop.
endloop.
Is this correct? I can't see anything complicated there - do you have a specific question?
cheers
Paul