If you mean a backup of Roon’s database, as handled by Roon’s Backup function, then you cannot use the 2.5" SSD installed in Roon as a backup location - it is dedicated solely to holding your local music files and folders.
Do you mean by your question “how do I backup the contents (music files and folders) of the 2.5” SSD?"?
In this case, you need to use a third-party backup or syncing application running on another computer or NAS.
Roon does not have a Backup function built in that handles music file and folder backup.