How do I keep track of the changes on my itinerary?

While you won't see an edit history look for a CREATED and UPDATED dates at bottom of an itinerary and a booking.

You can always make notes in the INTERNAL NOTES section of a booking.

AXUS Essential Webinars: Using Internal & Collaborator Notes
https://www.youtube.com/watch?v=JJeQqDrOc_w