Can anyone tell me how to UPDATE a column w/ an aggregate value (sum) w/in an UPDATE statement?

I receive the error: "An aggregate may not appear in the set list of an UPDATE statement" in the follwoing code:

update TabAAA set A.col_777 = sum(A.Salary)
from TabAAA A TabBBB B,
where (A.col_1 = B.col_1)
and (A.col_2 = B.col_2)