hey guys, i need some help here. I have a items in my database named as
product number | product name | dosage | description | price | remaining box(es)
0023 amoxicillin 50mg none 50 50

i am having problem in my buy button. how can i manipulate the value in the remaining box(es). for example, i select the item named amoxicillin, and i entered 5 on my textbox. when i press the buy button the value in remaining box(es) will be 45. can you guys help in this problem.