Where do I return the cable boxes and other equipment that was recently uninstalled? Do I take it to a store or to a service center?

Yes, you can return your boxes here as well as pay your final bill. You should keep it in case Comcast call you a few months down the road

At the store. Walk into any Xfinity store and drop it off

The store is a service center so you can return equipment there.

Yes. You can return equipment to the xfinity store fronts