How to update redshift table (t1) from another table (t2)?
You'd probably use something like:
UPDATE t1
SET t1.c = t2.c,
t1.d = t2.d
FROM t2
WHERE t1.a = t2.a
See also: Examples of UPDATE statements - Amazon Redshift
The table mentioned after UPDATE acts like it was included in the FROM and the extra tables can be added via the FROM, with joining via the WHERE.