Using Aggregate values in a update statment

I am trying to update a master table which has the following:
customer_no
fname,
lname,
FiscalYear,
attendance,
attribute1
..
Attribute25

I have at this point inserted all customers 3 times, each one having a fiscalyear = 2003, 2004, 2005

I am now trying to update the various attributes one by one with update statements like:

update lt_table
set attendance = isnull(SUM(CASE when sp.sp_act = 39
THEN 1 ELSE 0 END), 0)

from lt_table x, t_special_activity sp
where x.customer_no *= sp.customer_no
and x.fiscalYear = dbo.lf_fy(sp.sp_act_dt)
group by
x.customer_no, dbo.lf_fy(sp.sp_act_dt)
having dbo.lf_fy(sp.sp_act_dt) >= 2003

SQL server returns error:
An aggregate may not appear in the set list of an UPDATE statement.

Any ideas or paradigm shifts for this are much appreciated, I know there must be a much better approach to this project.

Thanks,

dj