Hi Mari,
Welcome to the community and thanks for your question!
If you are trying to update a web app you made on developer.amazon.com, you will just need to create a new version update to change the URL:
If you made your app on creator.amazon.com, unfortunately it’s not possible to change the URL at this time. You will need to create a new app.
Hopefully this answers your question. If not, please provide a few more details about what kind of app you have and exactly what you are trying to do.
Moses