Thanks for your question – this is an interesting one!
I did a little bit of testing, and when I wanted to change the name of someone in my Alexa app, I just changed the name in my phone book, and the Alexa app was instantly updated!
I’m not sure what make/model of phone you’re using, but make sure your Alexa app has access to your contacts (you may need to uninstall and re-install the Alexa app, then go through the Alexa app setup to confirm that access is set properly), but once that’s set, you should be able to change the name in your phone’s contact list and have it appear that way in the Alexa app.
Please report back how it goes!
-Daniel