Backing up in Lightroom

   Choose all photographs in your library panel.

  Goto file menu, and Export as catalog

  Give your catalogue a name.

   Uncheck export selected photos only, to export your entire catalog.

   Navigate to where you want to save your back-up, and click export catalog.

Lightroom will now create a whole back up of all your photos and also the catalog file.

If needed, you can open the back up by going to File>open catalog and loading the catalogue file.