Renaming your rails 5 app is even easier!
Open up the config/application.rb file within your rails project, there you see a module with the same name as your app. All you need to do is change the modules name to your preferred app name.
Besides that there are some other references to the old app name, but changing them is not required. Check out config/initializers/session_store.rb for example, where the session cookies name still uses the old app name. But the good old ‘Find all’ function in your IDE/editor can probably help you with these references.