I am using a LimeSurvey installation running on LimeSurvey's own servers (LimeSurvey.org).
I would like to change settings that unfortunately cannot be done through the GUI (increase the number of failed attempts to enter the token before it times out).
A forum search revealed that these settings are in /application/config/config-defaults.php, which can be adjusted via the config.php file.

My actually trivial question: How can I access the config-defaults.php or config.php file?

As a user of SaaS services you won't have access the server and thus you won't be able to edit config.php.

You will have to open a support ticket with Limesurvey GmbH (via the support link in the website) and see if they adapt this for you.