How to Increase and Decrease the Value in SQL Server

how to increase and decrease the value in sql server.in a table insert as product ,brandname, weight,price,purchaseno  in table1.

eg:

product : tooth paste
brandname :colgate 
weight:50kg
price:25
purchaseno:10

product : tooth paste
brandname :colgate 
weight:100kg
price:80

purchaseno:50

so am gng to sales 50kg paste 5.now i need the remaining paste as 5.
again am to purchase 50kg paste 25 now i need to add with remaining paste 5(25+5=30).i need to show 30 paste as available .