Good news for everyone struggling with the Medallion App. You can now update your info on the Princess.com web page. Go to your trip summary and you will see the following option, "Try our web version". I did everything in my browser. Saved my photo to my drive and uploaded it into the web page.
I did struggle to sign on using my login and password but there is an option in the bottom to sign in with name, birthdate and cruise number. That got me in immediately.
Eric