In order to select the desired editor, please go to the following location.
Masteriyo > Settings > General > Editor
Once you have selected the editor and saved the changes, we suggest you reload the page to reflect the changes.
Now, you can edit/create the courses with the selected choice of editor.
Course Details Editing
Similarly, you can edit the Lessons and Quizzes with the same selected editor as well.
Lesson Editing with Block Editor