Backup to external hard drive

You could use the "Store a copy of all originals" option in the Preferences>Local Storage tab. Though that really isn't intended as a backup (for one thing, they are the unedited originals).

You could export all the images from the cloud using the "Original+Settings" option, which will also include the edits.

Or you could use the Adobe Lightroom Downloader app, though that is primarily intended for those people who are terminating their subscription and need to download their cloud content....so not really intended to be used over and over again.