I want to do something really simple, but have to use Joomla what makes it probably complicated because I have less experience in Joomla development.

I have field with a value (a number) which is declared by another component and saved in the joomla database. Then I have a submite button in a form (I'm using chronoforms5 by the way). 

What I want is to change the value of my field after pressing the submit button of my form. 

Usually I would write a PHP file with the database connection, query etc. and wrting a javascript handler for the button which makes an AJAX call to that PHP file. But how can I do that by using Joomla? I'm searching for a simple solution, security is almost not relevant. Thanks!