To upload a backup file to the Plesk control panel's backup repository please follow the following:-
1) Login to Plesk control panel.
2) Click on "Tools & Utilities".
3) Click on Backup Manager.
4) Click on "Upload Files to Server Repository.
5) Click on "Browse" and select the file located on your local system that you want to upload.
6) Click "OK" and the backup file will be uploaded to the backup repository.