UPDATE Column values with Case and JOIN
Your CASE is almost correct, but to be able to do a separate join, you have to use a subquery:

UPDATE a
SET Material = CASE 
                 WHEN Element &lt;= 300000 
                 THEN 80000 
                 ELSE (SELECT b.Material
                       FROM b
                       WHERE a.PCOMP = b.PCOMP
                         AND a.Ply = b.Ply)
               END;