Here is how you enable it
To enable the Notebook Bar do this:
Click on the menu Tools > Options Select ‘LibreOffice’ > ‘Advanced’ Check ‘Enable Experimental Features’ Press ‘OK’ Atfer clicking okay you’ll be prompted to restart LibreOffice (yep the app, not your PC) so that the changes you requested can take effect.
Click ‘Restart Now’.
Once the suite pops back up you’ll instantly notice that things look no different to how they did before.
Head to the new View > Toolbar Layout menu. libreoffice view toolbar layout menu You’ll see a set of 4 toolbar layouts in LibreOffice 5.3: Default (Classic Toolbar) Notebookbar Single Toolbar Single Toolbar + Sidebar. Choose Notebook Bar.
That is how you enable the "ribbon" interface in LibreOffice. This has been tested on Linspire, Black Lab, Linux Mint, Ubuntu and OpenIndiana. We have NOT tested on Windows or macOS