When you're trying to update/make changes to a trip that happened in the past and is released. Please follow these steps:

Go on the trip page. Scroll down to the trip Checklist and go to the "Release Trip" section.

Click on "Update" 

Go ahead and Uncheck the released leg you want to update. If you want to update all legs, click on "Release All" and save the changes made. 

Go to the top right corner of your trip page to "Scheduled Itinerary" and now you're able to Edit the itinerary details on your past trip.