If you have an access to your Plesk server over SSH, you can manage Ruby versions via the command line. The commands must be run from the application root directory, that is /var/www/vhosts/<domain_name>/<application_name>, where <domain_name> is the web site’s domain name and <application_name> is the name of the Ruby application.

Plesk tries to keep in sync the selected Ruby version for the application. Run the following command to check the current Ruby version:

ruby -v

You can also use the  rbenv command line utility for Ruby versions management. For details, refer to https://github.com/rbenv/rbenv#command-reference).