Only the admin of the team is able to change the owner on an itinerary. The owner is the only person who can edit the itinerary (unless you are collaborating on an Enterprise plan. For more information, contact us here).
First, go to the team's itineraries and click on the trip you want to change the owner on.
Click on the current owner name next to the pencil icon, and choose which team member from the drop-down you would like, and select their name.