order table when making an update

I have the following table where order by priority ASC

 ----------------------
|priority |activity   |
|---------|-----------| 
|   1     |act1       |
|   2     |act2       |
|   3     |act3       |
|   4     |act4       |
|   5     |act5       |
|---------|-----------|

JSON where I make an update.

Add Method but it does not work as I wish

  <?php
      //update.php
      include_once('../include/conexion.php');

        $query = "
               UPDATE ACT_schedule SET ".$_POST["name"]." = '".$_POST["value"]."'
               WHERE id_schedule = '".$_POST["pk"]."'";
            $result=mysqli_query($conn, $query);

            if ($resultt) {
            $query2 = "UPDATE ACT_Agenda SET prioridad = CASE 
                   WHEN prioridad >= " . $_POST['value'] . "
                   THEN prioridad + 1 ELSE prioridad  END
                   WHERE id_agenda <> '" . $_POST['pk'] . "'";
            mysqli_query($conn, $query2);
            echo "YES";
        }  ?>

What I want to do is order the priority, if I update the act5 that has priority 5 to priority 1, the priority changes and that means that the priority of the act1 must change to 2 and so on until the act4 change to priority 5.

It works well if I update the last priority. But if I update the act4 to priority 1 the ones below should not be updated but they do it by adding +1 (act5 priority 5 is 6).

Something like that I would like if I update act4 to priority 1

   ----------------------
    |priority |activity   |
    |---------|-----------|
    |   1     |act4       |
    |   2     |act1       |
    |   3     |act2       |
    |   4     |act3       |
    |   5     |act5       |
    |---------|-----------|

I hope I explained well. Greetings.

from order table when making an update