WordPress just released it's latest version (5.0) today which is being widely discussed because some of it's major "improvements". While I appreciate the newer version, I really gotta admit that I did not like the new editor. Perhaps you are also one of those many individuals who did not like or appreciate the newly introduced editor (Gutenberg). If you want to get your old editor back. This is what you should do.
Step 1: Go to the "Plugins" section of your site and click on "Add New" option.
Step 2: You should see the "Classic Editor" plugin is already there from the window. If you don't see it then search for it.
Step 3: Download and Activate the "Classic Editor" plugin.
Step 4: Go to the "Settings" menu and click "Writing" option and you should see the following options available for you.
Step 5: Make the necessary changes as you desire and hit the "Save Changes" button located at the bottom of that page.
You are all set and you should be able to use the old editor as it was before. I chose to stick with the classic editor and allow my users to use the same. Hope that helps.