Prova detta med JOIN
UPDATE
tbl_a AS a
JOIN
tbl_b AS b ON (a.DIV_NO = b.Div_num AND a.OFR_ID = b.Flg)
SET
a.MIN_5 = b.Min_Five,
a.IN_PR = b.Ad_Flag,
a.FRC =
CASE
WHEN b.Prc_Fl = 'N'
THEN 'Y'
WHEN b.Prc_Fl = 'Y'
THEN 'N'
ELSE 'D'
END