Hi @Innovata
Upon updating X and Cornerstone you should be able to edit your sections and elements just as before.
If you confirm you have X and Cornerstone updated to the latest versions (based on your comments you do, but please double check)
If you have any cache plugins, please flush the cache and test it again.
If you have done that, please flush your browser cache and test it again.
If that still does not help, please try testing for a plugin conflict by turning off all third party plugins and test it again, if the issue is resolved to activate one by one until you find the conflicting plugin.
If that does not work, please provide your login credentials in a secure note so we can take a closer look on your issue.
Thank you