Here, bind primary key of database to datagrid column make 
it visible false.
and travel loop as 

foreach(DataGridItem dataItem in DataGrid1.Items) 
{
      priceText = (TextBox)dataItem.FindControl
("txtBookPrice");
   bookIdText = (TextBox)dataItem.FindControl("txtBookId");

Updatecode_for_perticular_row ();

   }

txtBookPrice= textbox from witch u would like to update
txtBookId= test i.e. unique id comming from database

With help of unique id in for loop  find perticular text in 
texbox and update.

This code you will write on update button click.

First you load 20 records to DataGrid and add one 
more templet field like"Chech Box" finally add one button 
in your page for update operation

        you write code whatever CheckBox clike get all row 
id's and you can update row based on selected id's