To edit config.php is extreamly easy, on Cloudamo, we offer this kind of support for free, but if you wish to learn yourself here steps:
STEP 1:
Login to cPanel, and access File Manager.
STEP 2:
Access to public_html folder and then access to config folder.
STEP 3
After that click on config.php and in top toolbar select EDIT, after opening you have full access to Nextcloud config.php
Full list of parameters you can find here: https://docs.nextcloud.com/server/latest/admin_manual/configuration_server/config_sample_php_parameters.html