Mopidy is configured using the mopidy.conf file. Where that file is located depends on how you run Mopidy and this is described in the documentaion at https://docs.mopidy.com/en/latest/config/.
The specific configuration for Mopidy-Local (such as the media_dir setting you want) is described at https://github.com/mopidy/mopidy-local#configuration.
I don’t think any of the Python 3 compatible webclients help you to configure Mopidy.