How i can update the table data through datagridview?

Question

i fill the datagridview with the students marks. ( i m using sqlserver as dbms)
now i want that if i do any changes in the data DGV , for example
if i edit the three students marks in datagridview and when i cilck update then only these three students data update in database not the whole students data which is appeared in the DGV. Means only  update the three row which i have edit.
how can i do this please help me?