How to get the latest version of CERN Override Theme
1. Copy paste your old version of CERN Override Theme in a safe location.
2. Download the latest version of CERN Override Theme following the instructions here.
3. Copy paste the content of your old theme.css file to the new theme.css which you mounted to your site (following the instructions in step 2).
4. Copy paste any other custom css file you may have.
You may also be interested to watch the recording of the Zoom workshop: Drupal 9 upgrade: Updating the CERN Override theme.
Questions or issues? Get in touch on the Drupal Community forums.