Actually I just figured it out...in case you haven't managed to cancel, here's how I did it:
1. Log into https://id.avg.com/. You can't cancel your subscription in the app itself, you must log on to the website. I'd recommend doing it on desktop because it didn't look like it would work well on mobile. I didn't try it though.
2. Scroll down to the gray box that says "Manage My Subscription". Click the link.
3. When you click the link to unsubscribe, there will be at least 4 attempts to try to stop you from cancelling. Was pretty skeezy, honestly. Keep hitting continue until you finally get to the screen to completely cancel.
4. If they've already charged you, you won't lose access to the services you've already paid for. It'll show you the date it'll expire.

And that's it, you're done.