how to insert value in a text box which is present on gridview into database

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ASP.NET

Actually i am using one gridview for displaying fee details in that it will display all the fee types,amount,amount to pay. and one text box will be present on the gridview of every row in that we have enter the amount and at the bottom i am having submit button if i click it should insert the fee details with entered amount into the table. below roughly i will show my gridview presentation

Fee_type     |amount  |paid_amount| amount_to_pay  | textbox
reg_fee      |1000    |  500      |   500          |  400
admission_fee|2000    |  1000     |   1000         |  5000
tutuion_fee  |2000    |  900      |   1100         |
transport_fee|1000    |  800      |   200          |  100

think that like above design i will display details in a gridview under textbox column i will enter the amount i will click on submit button i should insert the details. here my problem is i should save according to the fee the amount should be saved .in which gridview event the text box event will fire...