With the release of APEX 19.2 you couldn't really decide when Oracle would upgrade your APEX version in the Always Free Oracle Cloud. But this is now changed. From now on you can decide when Oracle upgrades your Oracle APEX version.
This is the email I received a few days ago:
If you do nothing your APEX instance will be upgraded as soon as possible, but you can decide to defer the update up to 45 days.
In order to check what the settings are on your instance, log in to your Oracle Cloud account.
Go to Autonomous Transaction Processing and click on your database, in my case DBDIMI. From there click on Tools and hit Open APEX:
You might have a direct link to your Oracle APEX instance, you can click on that too.
Login to your INTERNAL workspace, with the ADMIN user:
On the right-hand side it will tell you when an update is being done for Oracle APEX:
By default, it will update automatically when Oracle APEX is made available for your platform.
But if you click on the gear icon you can change it:
Here I changed it to defer the update, up to 45 days this can be done before it performs the upgrade automatically.
You find some more frequently asked questions in this blog post.