Variant 1: Setting the URL base in the UI

1. Go to Settings -> General
2. Set URL Base to desired path
3. Save settings
4. Restart (Restart as administrator on Windows)
5. Donate <3

Variant 2: Editing the configuration file

1. Open file ~/.config/NzbDrone/config.xml
2. Add entry <UrlBase>your/desired/path</UrlBase>