Updating Table With Running Total

Have a table where I need to update a field with a running total.

Table OrderRebate
I need to calculate a running total on ext_rebate for each order number. So I need the running total to reset to zero when the order number changes.

I've got as far with the code below but it never resets. Does a running total for every record.

DECLARE @runningtotal decimal(16, 6)
SET @runningtotal = 0
UPDATE OrderRebate
SET @runningtotal = RunningRebateAmt = @runningtotal + ext_rebate
WHERE ord_no = ord_no