Error in update statement - response (1) by M.Saeed Khurram

HI,
Try this:

UPDATE CUST_T
FROM (SELECT A.CUST_ID,A.CUST_DT
FROM CUST_T B
INNER JOIN
CUST__HIER A
ON B.LAST_CUST_ID = A.CUST_ID)  UPD1
SET
CASE WHEN IMMED_CUST_DT = '9999-12-31' THEN IMMED_CUST_DT = UPD1.CURR_CUST_DT END