I purchased it off
https://www.facebook.com/Bmwtechservices/
He sends through a file which you copy onto a blank USB. Insert the USB into the car for 40 seconds, remove, restart iDrive, wait a couple of minutes and you're done.
I was advised the full screen CarPlay only works with certain firmware versions - the company you're purchasing off should get you to confirm prior, and also provide your VIN.
After I installed CarPlay the first time, I took my car in for a service and they updated the software. It removed CarPlay and I had to reinstall it. I now request that no iDrive updates are installed when it goes in for a service.
Realistically I think BMW would have a hard time voiding the warranty for something like this. Probably worst case is your iDrive screws up and you need to install the latest software to revert everything to defaults and you lose CarPlay.
It's a risk you have to be comfortable taking I suppose.