Change a label value in gridview based on DropDownList

I have a gridview that gets its data source using the SqlDataSource tool. When clicked on edit, the GridView has Drop Down Lists for 7 columns. If the drop down list value changes then another column's value should change based on selection.

 

If the subject name is changed then the credithours column should display the credit hours as cumulative of all subject credit hours.

 

I couldn't find anything online. Can anyone help me with the code.

 

Thanks.