Edit Existing Post or Page Using WordPress 5 (Gutenberg)
In this video you’re going to learn how to safely edit those posts and pages that were created before WordPress 5.
Before you edit existing content created with the classic editor, it’s a good idea to create a copy of the post or page. If things don’t go as expected you’ll have the copy to fall back on.
After you’ve installed and activated the Classic Editor plugin, go to Settings > Writing and tweak the settings to look like this: