^^^ Need to change phone as Developer's preview device to get WP8.1 upgrade as this upgrade is only for developers 🙂
SVK said:By a tweak all current WP8 handsets can get WP8.1 developer preview.
Steps to follow are - http://www.wpcentral.com/how-sign-windows-phone-81-preview-developers
1. Create account for free in http://appstudio.windowsphone.com/ by signing in with same MS account which is used for Windows Phone.
2. Install Preview for Developers ass (http://www.windowsphone.com/en-us/store/app/preview-for-developers/178ac8a1-6519-4a0b-960c-038393741e96) and open app and accept T&C.
3. Now phone is ready to install developers preview OS and if new developer preview OS update is available, then check for updates and install updated OS.
4. This update would only include update from Microsoft and would not have firmware update from OEM (e.g. Nokia updates).
5. Official update with OEM firmware can be installed after update is officially release to all handsets.
Note: WP8.1 developer preview is expected to release this week (starting 14 Apr 2014) and I have not checked as of now if update is available. But if available, I will surely update my phone 🙂