Turning off app tracking on already-downloaded apps: How it is done.

For apps you’ve already downloaded that you may have tracking permissions set up for, you can still turn these permissions on or off on a per-app basis.
Under Settings, tap an app, then tap to turn off Allow tracking. Or go to Settings > Privacy > Tracking, and tap to turn each app on or off you’ll see in the list of apps that have requested permission to track your activity.
All app developers are required to request permission for tracking. If Apple learns that a developer is tracking users who have requested Do Not Track, they will either need to update their tracking practices, or they may face rejection from the App Store.
Apple believes that privacy features like these are a distinguishing factor in its products. Cook said that because the company’s business model is not built on selling ads, he can focus on privacy.
However, it’s important to keep in mind that when you ask apps not to track you, basically all they’re doing is preventing app developers from accessing the Identifier for Advertisers (IDFA) on your iPhone. The developers use your device’s IDFA to track you for targeted advertising purposes. Denied access to your iPhone’s IDFA doesn’t necessarily mean app developers won’t track you through other means, so it’s important to be mindful of the apps you use and how you interact with them.