U can't make a cell editable by just selecting it.
A row can be make editable by providing an Edit column.
1. Right click on GridView, Click on SmartTag
2. Click on edit columns
3. Add a Edit command button.
Hope this would solve your problem, if still have some doubt then pls revert back.
by using edititem property of Gridview we can do it
gv1.EditIndex=e.newIndex;//this code is under gv rowediting event
or
with the help of smart tag we can do it easily this is automatic way