Earlyface

Flashing a WP8 Nokia Lumia phone is explained in this tutorial. Thanks to Nokia Care Suite (NCS) you can flash your phone yourself at home with any valid firmware you like, be it stock or custom (modified). This is also helpful for flashing a firmware meant for a different RM on your phone, thus de-brand it and add additional RM-specific features to it.

Obtaining Firmware Files:

• Get Navifirm+ from Attached below.
• Open it, select ‘Nokia Care Suite’ from the dropdown menu in top-left.
• You’ll see a list of phones on the left. Select your phone.
• You’ll then see the available softwares for it on Nokia’s server. Select the one you want to install. (REMEMBER : Downgrading may kill your phone, so always flash either the same firmware version your phone is on, or newer)
• You’ll see the available variants of the update. Select any! and download all the files.
• Place the files in the directory : “C:\Documents and Settings\All Users\Application Data\Nokia\Packages\Products\[Your Phone RM]\”

NOTE: Check your phone’s RM on the box label.

Flashing Firmware Files:

• Download Nokia Care Suite 5.0
• Open it, click on File->Open Product, select your phone.
• Right click on the ‘Programming’ button on the left. Click on ‘Recovery’.
• Click ‘Update List’, and select the firmware it finds.
• Check ‘update automatically’ and ‘USB dead phone flashing’ and click ‘next’.
• Follow the on-screen instructions and turn your phone off.
• It’ll say ‘application has lost the connection with your phone’. Turn your phone on at this point holding the volume-up button and quickly press ‘retry’.
• You’ll see the flashing procedure starting and it will be finished within a few minutes and your phone will be ready to use.

NOTE: DO NOT FORMAT your device's internal storage if your PC asks to do so. Just ignore the message and go on with the flashing procedure. Formatting will brick it, and in that it'd be pain to revive it back without taking it to a Nokia Care centre.

This way you can flash a RM-821 Lumia 920 with RM-820 software or vice versa. Though phones can be debranded using this method, do keep in mind that in some cases, due to the change of basebands, telephony (calls) doesn't work. If that happens, just revert back to your original firmware.