Here's the query...

update temp_schedrcpts u
set manu_item = (select imlitm from F4101 x, temp_schedrcpts u
where x.imaitm = u.legacy_item)
where exists (select 1 from F4101 x
where x.imaitm = u.legacy_item);


NOTE: the temp_schedrcpts table has a row where the legacy_item = 'XYZ' and the F4101 table has a row where imaitm = 'XYZ'