Hi,
If you wish to restore the database backup from the ones you have taken from Plesk. Login to your Plesk panel & access option Backup Manager . Click on the backups from which you wish to restore the backup. From the option ' What do you want to restore?', tick the radio button for Selected objects & select Database from dropdown for 'Type of object to restore'. From the Available, select the appropriate database and then click on restore by selecting the correct Restoration options .
That's it. :)