ASP.Net GridView Bulk Edit and Update Rows

here is aspx code

i have a form in which when i enter value and upon pressing submit button all the data were shown

on the grid.from where i can edit and update the grid view.

in this case i can edit the only particular row.

but i want to edit the whole grid and then update it. means bulk edit

can any one please share the code regarding bulk edit and update according to my 

above mentioned code.

help me any body