insert update delete in gridview

I am doing a school software project. I reterive all the student names and subjects
from that class in a GridView.
Based on that, for each subject I want to enter the marks and calculate
the total and average within the GridView and insert it in to SQL Server database.
Problem is that I don't know how to edit values inside the GridView. If anyone have
idea inform with some code examples.
 
Thank you