Finding index of cells of underlying collection of gridview's datasource using LINQ

Finding index of cells of underlying collection of gridview's datasource
using LINQ

I have a gridview which is binded using BindingList<>. I am trying to
change some values to grid's cell. So, I extracted the list and trying to
find the cell using the where conditon in linq where i made change, but no
success.
Graphically,it is
Col1 Days col3 DateStart DateEnd
----------------------------
1 14 3 12-12-2013 25-12-2013
5 4 3 06-03-2013 09-03-2013
Update:
In the above table, if i change the days, then end date will get increased
by the no of days suuplied. That is , in row 2 my date is 6th march - 9th
march which is 4 days difference{see days column it has 4}, if I change it
to 6 then the new end date will 11-03-2013 (dd-mm-yyyy)