You can ask the Educator Agent to change a lesson you already have, rather than editing it yourself.
Asking for an edit
Open the lesson and click Agent in the header, which opens the agent alongside the lesson it is working on.
Ask for what you want: "add a recap page at the end", "make question 2 harder", "write a mark scheme for the last question".
In the New editor the agent proposes its changes for you to accept or decline, so nothing lands in your lesson without you seeing it.