How To Downgrade WordPress Core Version
You can downgrade to a different WordPress core version by installing a plugin called "WP Downgrade | Specific Core Version" by Reisetiger. Before moving onto the steps, we strongly encourage you to backup your site. Please refer to the article here about how to do so.
Log into your WordPress Admin and go to Plugins > Add New. Type in WP Downgrade in the search box.
Click Install Now on the WP Downgrade. Once installed, click Activate.
Go to Settings > WP Downgrade
Type in your target version of WP (ex. 5.4.2), then click Save Changes.
Go to Dashboard > Updates > Re-install Now
That's it. Check your site and make sure to delete the plugin when done.