Each time you open a Lesson as an Author, it creates a new Revision of that Lesson. All Revisions, including past ones are always available to view and Restore at any time.
To revert to an earlier Revision, click the 'Revisions' button on the Lesson card (Lessons > Lesson of Interest > Scroll Down > Revisions) and you will see a list of all the different Revisions for your Lesson:
You can open these (click on the one you want to View from the list) to see which one has the content you want. Once you've chosen which revision you want, click 'Restore'. This will make that Revision the 'current' Revision (idicated by a 'tick' next to the Revision)