How to create a GridView edit button manually and write code for it?

Add this code in you gridview columns.
Add template field in your gridview.

<asp:TemplateField >
                                       <ItemTemplate>
                                           <asp:Button id="btnEdit" runat="server" Text="Edit" CommandName="Edit" CssClass="btn" CausesValidation="false"/>
                                       </ItemTemplate>
                                   </asp:TemplateField>
                                   <asp:TemplateField >

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In Codebehind

protected void gv_RowEditing(object sender, GridViewEditEventArgs e)
  {
      Int32 iID = Convert.ToInt32((gv.DataKeys[e.NewEditIndex].Value));
      EditMethod(iID)         // now you can send ID and retrieve data.
  }

Please write this code in code behind.

You can get Id of editing element. This can fetch data of specific row.
This is final solution.