Thank you for writing in, first please make sure that your X theme validated.
Then update the X theme first, If you don't see the X theme update notice, please navigate to Admin Dashboard > Updates and hit the "Check Again" button.
Updating Your Themes and Plugins
If you have updated the X theme, you can easily update the Cornerstone by deactivating and deleting it. And then navigate to Admin Dashboard > X wait for a few seconds and X will automatically install a compatible version of Cornerstone.
Remember to clear all your caching features (plugin, server-side, CDN, and browser's cache) after updating so that the code from the latest release is always in use. This will help you to avoid any potential errors.
Hope it helps,