When you click on My Device > 3 dots > Select Device, you can add a new device, but it doesn't seem possible to remove a device. I suppose uninstalling and reinstalling Connect IQ might remove all devices (although I haven't checked), but it would be nice to have this feature in the app itself, for when you buy a new device and you want to "retire" the old one (from the Connect IQ app and not necessarily anywhere else.)
