Assuming you have a local database instance setup or use local db to get the job done.
Write a SQL command to do an update to a 'Total' table then re-bind to front end controls to show updated value.
e.g.
Table: 'Total'
___________________
|ItemId|Qty|OrderId|
| 1| 2| 1|
|______|___|_______|
SQL:
(I am assuming you are using MS SQL express or something like that)
UPDATE TOTAL SET Qty = @Qty WHERE ItemId = @ItemId AND OrderId = @OrderId.
Pass in some SQL parameters with the '@' prefixed names above and set their values to the new Qty and item/order id.