Configuration → Backup/Restore
To backup the complete configuration data, just press the button. After a few seconds (in which the archive is compressed to save space), your browser will download an archive with the device settings and all user data.
When restoring a previously generated backup archive, you can choose what parts of the archive you whish to restore: Only the device configuration without replacing all the user accounts and data, only the user data, or both.
Depending on your choice it might be possible to either reboot the device or log out and back in.
Warning: Replacing the user data also overwrites the administrator's account. Make sure you have all the authentication credentials at hand!
Note: If you replace the device's configuration, it will behave exactly the same way it did when the backup was made. That includes the network configuration. If you cannot find your device again after the reboot, it probably uses an old network configuration. See the section about recovery on how to deal with that situation.
This description says it all: If you want a fresh start or give back a rented device, press this button. A confirmation dialogue will make sure you didn't click by accident.