How do I update to IOS 16.0 only? I can’t get latest one because it’s unsuitable for my diabetes app

It's not something I normally recommend. But if the phone you use for your app works now, don't update it until the app works on the latest version.

Disable Automatic Updates so your iPhone doesn't update on its own. Go to:

Settings → General → Automatic Updates → disable Download iOS Updates

Unfortunately, it is not possible to update to versions before iOS 16.3.1 on supported phones. It is also not possible to downgrade. If the app you are using does not work on iOS 16 or later, you can try contacting the developer of the app and ask if there is an update on the way. Just for this kind of app, it is important to keep it updated, or you should look for alternative apps.

You're welcome.