datarepeater edit a certain row

how do i update a textbox in a datarepeater on runtime,

I have a datarepeater with a button Update when user clicks the button another form open to update .. the update was successfuly done but when i close the popup form i need to update the row index of the old value of the textbox without populating again from datatable and bind the datarepeater..

Or how to update inside a datarepeater please give me tutorial  or sampme walkthrough  if this can be done