To start, log in to Azure portal, navigate to the web app(s) go to Configuration > Settings > Stack Settings. You will see dropdowns for the Java major and minor versions, as well as the Tomcat version (if you are using Tomcat).
You can also view other resources below before you upgrade your Web Apps to Java 11 or 8.
If you have any additional queries regarding this retirement, please use comments on this thread to ask your specific queries and we will try our best to answer those queries.