Currently, we provide backup files for up to 30 days for Premium users and 7 days for Basic users. You can select the backup from a specific Address Book or for all Unified Contacts. To get the file, click on Download for the appropriate date, this will download a .zip file containing a .vcf file of your contacts.
(If you need backups from over 7 or 30 days, please contact support and we can provide those for you)
To import the backup file into your current Unified Contacts, see here.