You update to WordPress 5 and notice that the WYSIWYG looks a little different. If you aren’t quite ready to update to Gutenberg, it’s quite easy to revert it back to the standard WYSWIG editor. Inside of your functions.php file simply add the following:

/**
 *   disable Gutenberg for posts
 * 
 * */
add_filter('use_block_editor_for_post', '__return_false', 10);

/**
*   disable Gutenberg for post types
* 
* */
add_filter('use_block_editor_for_post_type', '__return_false', 10);

Log back in to your WP backend, and things should be back to normal. If you only want to disable it for ceratin pages or posts types, that’s possible too, and this article has a great round up of all the methods.