App updates

If you are looking for a feature of the App Store to designate an app update as “required” (meaning the OS would automatically block your old versions from running or automatically install the update) then there isn’t one. The user always has control over app updates.

Of course you can implement your own app logic to (for example) query a web API to see if a new version is available, and then degrade your app's functionality, such as displaying a message inviting them to update. But think really hard before doing this: it’s unexpected behavior and a terrible user experience. And of course you would need to have built this version checking functionality into whatever previous version of the app the user already has. Which, since you’re asking the question, probably isn’t the case right now. It’s best to do whatever you can on your back end to keep old versions working as long as possible.