To create volume backup you need to umount it first in Linux:

# umount /dev/vdx

where x is the corresponding letter of your device name.

To make Backup option avaliable you need to detach the Volume in Horizon.
Go to Volumes and choose "Manage Attachments" from the scrolldown menu.

Then detach your Volume by clicking "Detach Volume"

Create backup from Horizon menu Volumes -> Edit Volume -> Create Backup.
 

After creating a backup you will see it listed in Volume Backups menu:
 


How to restore the backup:

Before restoring Volume Backup volume should be unmounted in Linux and detached in Volume menu in Horizon.

The Volume Backup is non-incremental and will overwrite every data that was writen to the Volume after backup creation time.

Your current volume will be replaced by the backup data.

To restore the backup choose the option "Restore Backup" and confirm:
 

 
After restoring the Volume mount it in your Linux.