Avada DOES NOT AUTOUPDATE ANYTHING ON YOUR SITE. If you’re seeing something on your site that updated without you initiating the update (not just an update available notice), check your cPanel settings, backup plugin, or WP management plugin. To answer your specific rollback question – steps are pretty straight forward:
1) Change the active theme setting in WP>Appearance to one of the WP defaults,
2) DELETE (NOT RENAME!) the Avada folder from ~/wp-content/themes/ and the matching required Avada plugins (fusion-core & fusion-builder) from ~/wp-content/plugins.
3) Reinstall the earlier version of the Avada theme in the theme folder,
4) Activate it through the WP dashboard (WP>Appearance),
5) Reinstall the required plugins from the WP>Avada>Plugins page
6) Install the patches from WP>Avada>Patches
NOTE: Don’t forget to clear all caches post rollback (server, site, and browser) after you’ve completed the above. Steps tested on our dev server from v5.5.1 back to v5.1.6, running WP v4.9.6.
Whatever your reason for wanting to do a rollback, long term it’s better to fix the problem then roll back to an earlier version. Avada Support really will help and respond quickly, plus if it is a problem on their side it gets documented… and fixed.