To make the gridview editable, you can use Gridview's
OnRowediting
event.
that can make your task really easy and understandable,
to understand this event Kindly have a look at the following link.
GridView.RowEditing Event
and to understand how you can implement this. Do have a look at the following article.
Editable GridView in ASP.NET 2.0
Hope it will help. :)