Installing Classic Editor
1. Start WordPress and login to it
2. Search "Classic Editor"
3. Click [Install] and [Activate] button
Make Classic Editor Default
1. Click [Settings] on Classic Editor of plugins' page
2. Choose Classic Editor as a default editor for all users.
Make Classic Editor default, choose [Classic editor] on [Default editor for all users].
Choose [Yes] on Allow users on [switch editors]
That's all for make Classic Editor default editor.