From Lunarpages Web Hosting Wiki
Backing up your web hosting account is one of the most important things you can do to protect yourself when the worst might happen. Just in case you over write a file that should not have been over written or you write a little of the wrong code, we have the backup tools you need in Plesk.
How to Backup Your Domain in Plesk
The first thing you will need to do is login to Plesk and get yourself to the front page. Once you have made yourself there, click the domain name you wish to backup.
Click on the icon labeled "Backup" to be taken to the backup files repository. You can backup locally or remotely. For convenience, you can enter your FTP details and store them for future sessions. To do that, click on FTP Account Properties. On the next page enter your FTP details. Double check to make sure that what you entered is correct, and then hit the "OK" button.
Now that you have gotten this done, we can start backing up your files. Go back to the backup files repository and click on the icon labeled "Create Backup". Now, all you have to do is fill in the boxes, give a name to your backup and then click the "Back Up" button. This domain will be fully backed up and ready to store on your PC just in case the worst might happen.