Hi Tasmin,
This is a fairly old version of X. For good maintenance of your website and to avoid problems we recommend that you keep your website always updates to the latest version or at least do not leave such a gap between the updates.
At this time you will need to perform a manual update, since this is a very big version jump it is possible that some issues occur, if that happens, reach us so we can guide you, in any case, we do recommend you perform a backup before this process.
- Revoke your URL from your Themeco License page and re-assign.
https://theme.co/apex/licenses
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Temporary disable any caching plugins and purge all your server cache.
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Delete both X theme (/wp-content/themes/x) and Cornerstone plugin (/wp-content/plugins/cornerstone) by connecting to your FTP (your settings will be saved in the database)
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Download the latest copy of X and upload it to your themes folder via FTP OR you can upload the x.zip via Appearance -> Themes section.
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Latest version of Cornerstone will be installed automatically when you head over WP Admin > X section. Now you try validating your X theme again.
Let us know how it goes.
Thanks!