Lumia Users! Yes, I confirm, that you can use that manual too, but your phone must have SD card slot (and actually SD card, of course) to edit the registry! Without - you cannot. Sorry. Don't use my method, if you model already have 8.10.14226 or 8.10.14234. Simply install Windows Insider and get your Update 2.
I have a Lumia 520 and this doesn't work. I followed the steps to the letter and it says that it can't retrieve xap from SD even though the Windows app is authorized to read from my SD. Plus I have preview installed, dev unlocked my phone, even deployed the app onto my phone via the SDK tool. Sorry dude it don't work.