How to Assign value to cell of selected rows of gridview pag

Hii,

   I am new to gridview paging .Here i have gridview with checkboxes,and 4 other coulmns

Document No,Date,Date and LotNo.Initially LotNo column is empty.When i will click on assignLotNo button then for only selected records i have to assign lotno to cell.i.e for every selected rows  cell[4].Here problem is that when i am going to do like that it will assign to lotno to  selected  rows cell eigther one of the page .i.e i have selected 2 rows from 1st page and 3 rows from 2 nd page then it only assign lotno on 2 nd page selected rows cell[4].

 

 

Please Find Attachement for my sapmle code. 

Please help me.