Is it possible to downgrade Plesk, certain Plesk components or roll-back Plesk updates?
Plesk does not support downgrading by installing the version you had before over the existing version.
If upgrade was performed for evaluation purposes and it is needed to have the option to roll back to the previous version, it is required create a full server backup before upgrade using Plesk Backup utility or create server snapshot using 3rd-party backup tools.
To roll back to the previous version, perform a clean installation of the Plesk previously owned and restore the full server backup or restore server snapshot made by 3rd-party backup tool.
If backup was not prepared and downgrade is required, the only option is to re-deploy the server and install required Plesk version from scratch.
Warning: All data will be lost in this scenario.