Gutenberg is included in WordPress 5 which is slated for release on 6th Decmeber 2018. Hopefully you know what Gutenberg is and if it will work for you.
If Gutenberg won’t work for you, or you haven’t finished testing, then you have two options.
Unless you’re on a hosted WordPress, in which case you may only have one option.
The first, or possibly only option, is to revert to the Classic Editor by installing the plugin. This will allow you to keep the current editing experience (until the end of December 2021).
However, if you’re using self-hosted WordPress, or otherwise have access to your wp.config file, you can add a line which prevents all core updates:
define( 'WP_AUTO_UPDATE_CORE', false );
One that line is added, WordPress will not update; this will allow you to remain on WordPress 4.9.x until such time as you have made a decision as to the route you’re going down.