Release notes are shown:
Within the app build history
Withing the e-mail notification if new app versions are uploaded
On the mobile download page of your app
There are several ways to manage release notes of an app version / build.
When you upload a new app version you can add your release notes.
If you would like to change release notes of an already uploaded app version/ build you can select your app version within the app build history and click on edit release notes.