create table 'usudes911'.t (
a VARCHAR(2),
b VARCHAR(2)
)


create table 'usudes911'.t2 (
a VARCHAR(2),
b VARCHAR(2)
)

Suggested by IBM Informix manual (IBM Informix Dynamic Server v10 Information Center)

UPDATE t SET a = t2.a FROM t, t2 WHERE t.b = t2.b

Result...

UPDATE t SET t.a = t2.a FROM t, t2 WHERE t.b = t2.b
# ^
# 201: A syntax error has occurred.