Increment values in gridview column

Question

In my gridview which is in update mode, I need to increment a column value.

Ex: In my gridview have 5 rows and three columns. I need to increment the serial number column value by 1. Increment Gridview1.Rows[1].Cells[2] value by 1 up to 5th row. But when I execute the code it will result the output put of all the column same. This is my code.

 {

    int totalRows = GridView1.Rows.Count;

    for (int RowIndex = 0; RowIndex < totalRows; RowIndex++)

    {

        GridViewRow row = GridView1.Rows[RowIndex];

        TextBox txtslno = row.FindControl("txtslno") as TextBox;

        int number = int.Parse(TextBox5.Text);

        if (GridView1.Rows.Count > 1)

        {

            for (int i = 0; i < GridView1.Rows.Count - 1; i++)

            {

                txtslno.Text = (number + 1).ToString();

                number++;

            }

        }

    }

}