We have changed the validation system and now there is a big gap between the versions, so in order to validate you will need to perform a manual update. You wont lose your content as it is stored on the database.
So access your site via FTP
remove x from wp-content/themes
also remove cornerstone from wp-content/plugins
Re-install X via FTP or WP Dashboard.
Re-insert your url to and copy the license code https://theme.co/apex/licenses
Access WP via Dashboard and add the license code to Wp Admin | X
Hope it helps!