Install windows 8 on pc ,get publisher id (via preview for publishers app) n then install .xap through windows app studio from pc via,connecting ur lumia
A few steps:
1. Go to https://www.dreamspark.com/Account/CreateAccount.aspx and create an accout.
2. Don't need to be verified. You just have to register.
3. Download SDK mini tool Zippyshare.com - WP8 SDK Tools Lite Setup Av1.0.zip and install it.
4. After the installation your PC will restart. Than type in the search box Phonereg and open phone registration app.
5. Now connect your cellphone to PC via USB, unlock the lockscreen and turn on the cellular connection or WiFi connection (it
s necessary).
6. Now Phone Registration app will find your cellphone and you will be able to hit "Register".
7. That's it. After your phone is registered you can sideload the apps.
Do you want to unlock your Nokia Lumia 820 from carrier lock ? If yes, use unlock code to unlock it . You can get it from any of the online vendors like OnlineGSMUnlock.com to unlock it .