doubt in adding data in database in c#

Question

Hi... I am new in C#. I am doing a billing project. I have one doubt. In my form i have text box. for sales entry. when i enter in that text box it have to be added to my database in the field of sales. For example in my database for an item pen it already have sales value 5 in database. if i enter again 5 in text box my database value should be 10. I don't know what to do... so please help me