Right now, the Travefy Itinerary Viewer app is built for to view your itinerary only. You won't be able to edit on the app, however, you can edit on Travefy.com via your web browser and your app can help you do that.
When you are in the trip itinerary you would like to edit, you can press the Upload button at the top next to the conversation bubble.
When you press that button, you will have multiple options, choose Edit Itinerary in (your selected web browser).
You may need to login to your Travefy account on the web browser if you have not already done so. Once you are logged in, you can edit any trip information you would like as well as access other features like Expense and Discover.