Retrieving Files From Backup Archives
To restore your data on a Plesk 8 machine, use the pleskrestore utility as described in the Restoring Your Data (on page 10) section of this guide.
Plesk for Linux stores backup files in compressed GZIP archives. Only the server administrator with access to server shell can unpack these archives.
To unpack a backup file:
Log in to the server and issue following command:
/usr/local/psa/bin/backup-unpack extract , where is commonly /var/lib/psa/dumps///
The utility will create the directory …
.unpacked in /var/lib/psa/dumps/
… and will place all content there.
Reference: plesk backup and restore guide