How do I update a column that has the same ID as one of the form fields?

I want to update a column in a table that has the same ID as one of the form fields - with the value from another form field. How should I think here?

Apply the update server behavior and set the lookup column from the one form fields and bind the value from the other.

Okay, done!
Now the value is written in the column. But I want to add the "++" value to the existing value in the column.
How?

I'm afraid I don't understand exactly what you are trying to do. Maybe give an example of what the value would be before and after.

Sure. If the existing value in the column is 20 then this should be added with the value from the form field. For example, 4 + 20 = 24.

You could add the values with php in a hidden form element, or use the "Execute SQL" server behavior to manually write a SQL statement that does that like:

UPDATE tablename SET column = column+4 WHERE IDColumn = value

So it is not possible to work with calculation when adding or updating a column?

You could do it in the bindings panel of the update as well. It is very possible. There are at least three ways to do it. I don't know the specifics of your application so I can't comment on which one if any is preferable.

What is your current binding in the update panel? You can likely just put +4 after it to add four to the value you are currently setting it to.

Late update:
I've tried to write + in the binding panel as you said, but it doesn't help. It still only inserts the value from the column, and does not add it.
Here is the updated code:
$UpdateQuery->bindColumn("ProvningPlatserBokade", "i", "+".($WADABokning_update->getColumnVal("BokningAntal")) ."", "WA_DEFAULT");

Earlier:
The idea is that admin should be able to see how many places are booked for the specific activity. Each time a booking is made, the number of seats is added to the column associated with that activity.

The number 4 was just an example. Actually, it's a number that comes from another column in another table. So different from time to time. Here is the code from update server behavior:
<?php
if (isset($_POST["Update"]) || isset($_POST["Update_x"])) {
$UpdateQuery = new WA_MySQLi_Query($AboutBeer);
$UpdateQuery->Action = "update";
$UpdateQuery->Table = "Provning";
$UpdateQuery->bindColumn("ProvningPlatserBokade", "i", "".($WADABokning_update->getColumnVal("BokningAntal")) ."", "WA_DEFAULT");
$UpdateQuery->addFilter("ProvningID", "=", "i", "".($WADABokning_update->getColumnVal("ProvningID")) ."");
$UpdateQuery->execute();
$UpdateGoTo = "";
if (function_exists("rel2abs")) $UpdateGoTo = $UpdateGoTo?rel2abs($UpdateGoTo,dirname(__FILE__)):"";
$UpdateQuery->redirect($UpdateGoTo);
}
?>

$UpdateQuery->bindColumn("ProvningPlatserBokade", "i", "".($WADABokning_update->getColumnVal("BokningAntal")+1) ."", "WA_DEFAULT");