Well, it would seem that you would want to store the total in the table where only Ent_ID is the key, but I'll give it to ya anyway :)

UPDATE
--Temp T1
SET
--T1.Amt2 =
----(
----SELECT
------SUM(T2.Amt2)
----FROM
------Temp T2
----WHERE
------T2.Ent_id = T1.Ent_id
----)

HTH,

- Chris