You don’t make any changes to the “Live” app. All your changes will be updated automatically on Preview, but if you want them to be made available on the URL you showed above, you will need to Deploy your app.
If all you want to do is show the app to your partner, you can send them the Preview link and they’ll see whatever the current “in-development” state of the app is.
If you do a push to Live, that will not get updated by your progress unless you specifically perform another deploy. That means you can continue working on the app without worry that you’re breaking something on the Live site. Make sense?