I'm unable to change the navigation items on any Lightning (Managed) app, however it does work for Classic (Managed) apps.
Steps to reproduce:
- Go to Setup > App Manager
- Edit any Lightning (Managed) app
- Select "Navigation Items".
Note that there is a "closed lock" icon on the right hand side, however it's possible to add and remove any item available on the left hand side.
Is there a way to unlock it? If not, is there a simple way to clone an app?