1. Clear the site cache and the two logs (Error Logs and Manager Actions under the Reports menu)
2. Flush Permissions and Log Out
3. Backup files and database through cPanel
4. Use CPanel to delete all the files in the cache folder (core/cache).
5. Download the latest version of MODX Revolution, unzip, and merge (not replace) with the current installation (folders: connectors, core, manager, and setup) via FTP client
6. Change setup directory permissions (no sudo is required)
   cd public_html
find setup/ -type d -exec chmod 755 {} \;
find setup/ -type f -exec chmod 644 {} \;
7. Go to law.gmu.edu/setup. Click next, next, next (maybe one more next) and that should be it.

Source: Upgrading MODX
Also check out Bob Ray’s guide for upgrade.