Help !!!!!using case statements in update

Hi,
I have this update statement that works, it updates the totalamount to calc amount, but I want to update totalamount only when it is not equal to calcamt.I have tried many things but in vain.Can some one please help me.
How do i use case statements to update only when totalamount!=calcamt.

update c
set totalamount= calcamt
from Prepay c
JOIN
( select sum(amt) as calcamt, payid
from pay
group by payid
)b ON b.payid= c.payid
where c.cust_no='somenum'

Try this:

Code Snippet

update c
set totalamount= calcamt
from Prepay c
JOIN
( select sum(amt) as calcamt, payid
from pay
group by payid
)b ON b.payid= c.payid and b.calcamt <> c.dailytotalamount

where c.cust_no='somenum'