SQL - Add Data to Existing Data From Another Table

I have a temp table @table in my stored procedure that looks like this:

AgtID | Bonus
-------------
5063  | 0
1104  | 0

And a table bonus that looks like this:

AgtID | Contest | Points | Event
--------------------------------
5063  | 21      | 1000   | 1
5063  | 21      | 3000   | 3
1104  | 21      | 1000   | 1
1104  | 21      | 500    | 2

And an update query (currently) like thus:

UPDATE tt
SET Bonus = b.Points
FROM @table tt
INNER JOIN tblContestData_Bonus b
ON tt.AgtID = b.AgtID
where ContestID = 21;

Currently, when I run this query, it'll overwrite whatever data is in @table.Bonus with the data in bonus.Points. This is fine for a single record, but as we start getting more bonus point events, I need to have it add to my value.

I know some languages have a i += value... does SQL have something similar?