You can open regedit, select HKEY_LOCAL_MACHINE and use [File]->[Load Hive...] to load [Phone]:\windows\system32\config\system. Next you'll be asked for a name for the Hive. Name it something obvious like "PHONE_SYSTEM". Next Navigate to HKLM\PHONE_SYSTEM\Platform\DeviceTargetingInfo and change
// edit: Use values at your own discretion. The values suggested here will give you a phablet interface.
PhoneManufacturer to "Microsoft MDG"
PhoneModelName to "Lumia 1520"
PhoneManufacturerModelName to "RM-938_1000"
Unload the Hive by selecting HKEY_LOCAL_MACHINE\PHONE_SYSTEM in the tree view and use [File]->[Unload Hive...]. Press and hold [Powerbutton] and [Volume down] to reboot your phone to normal mode.
Join the Fast Ring using Windows Insider tool and check for updates.