A standard SQL approach would be

UPDATE ud SET assid = (SELECT assid FROM sale s WHERE ud.id=s.id) 

On SQL Server you can join a

UPDATE ud SET assid = s.assid FROM ud u JOIN sale s ON u.id=s.id 

A standard SQL approach would be

UPDATE ud SET assid = (SELECT assid FROM sale s WHERE ud.id=s.id) 

On SQL Server you can use a join

UPDATE ud SET assid = s.assid FROM ud u JOIN sale s ON u.id=s.id