OK, the backup file is named 2026-05-15_pleskbackup-<username>.tar
I unzip it and go into the folder called "domain" and then the folder named 'YourAccountUsername.helioho.st'
in there I find 6 different .tzst files, and .xml file and a folder called ".discovered" which contains yet another subfolder with more files
One of the 6 files in the folder named 'YourAccountUsername.helioho.st' is the file named 'backup_user-data_##########.tzst'
As I follow the instructions in https://wiki.helionet.org/Account_Backups#Access_Your_Files I open the file named 'backup_user-data_##########.tzst' and find a file with the same name, but without any extension.
With that it seems I have access to my user data, but I am still confused as to where I upload this in Plesk.
The only instruction I seem find: "To restore a backup, you must first extract the backup file on your local machine, then upload the specific parts you need". Yes, but how upload what parts where in Plesk?
Thankful for your support.