This is My Solution:
protected void GridView1_RowEditing(object sender, GridViewEditEventArgs e)
{
txtid.Enabled = true;
txtid.Text = GridView1.Rows[e.NewEditIndex].Cells[1].Text.ToString();
txtname.Text = GridView1.Rows[e.NewEditIndex].Cells[2].Text.ToString();
txtage.Text = GridView1.Rows[e.NewEditIndex].Cells[3].Text.ToString();
txtaddress.Text = GridView1.Rows[e.NewEditIndex].Cells[4].Text.ToString();
}