It used to be that hitting the refresh arrow/loop button in preferences -> account would pull down a new provisioning profile with all ur new devices.
Now it just downloads the ones you already have. But if the one you need updated is managed by Xcode, theres's no way I can find to add a device to it!
Here's what worked for me on Xcode 8 & 9.
My account on developer.apple.com was not showing any Xcode managed profiles so this was the only way to make it work.