If you have a Windows Phone 8 device, you can get your hands on the new Windows Phone 8.1 preview for free if you sign up for a developer account.
Use the following instructions to update your device!
There are a few key items that you must understand before you update your phone:
Sign up for a Developer Preview Account
Note: Perform these steps on any computer, these do not have to be done on your Windows Phone
Register as a developer with App Studio – Free, and create apps to run on your own Windows Phones
Click "Start New Project"
Sign-in with the same Microsoft account that is registered on the Windows Phone you wish to upgrade
Agree to the access request
Agree to the Windows Phone App Studio Agreement.
You do not need to proceed any further at this stage, you should be prompted with a request to select a template for your first application.
You should also receive an email that welcomes you as a Windows Phone app developer.
Feel free to develop some apps, but you do not need to do so to get access to the Windows Phone 8.1 developer preview… simply progress to the next steps below J
Install Preview for Developers onto Your Windows Phone Device
Note: Perform these steps on your Windows Phone
Download the Windows Phone 8.1 update package on the Windows Phone you wish to upgrade
Or search the Microsoft Store for "Preview for Developers"
Install the application
Launch the "Preview for Developers" application.
Accept Terms and Conditions
Login with the Microsoft Account (MSA) that you previously registered as a developed account and is the same MSA that is registered on this device.
Check the "Enable Preview for Developers" checkbox and click done…
You should see the following screenshot if all has gone well J
Open the settings app on your Windows Phone Device, Open Phone Update and tap Check for Updates
Note: You may have to go through this process a number of times as updates are applied. Follow each updates instructions and after each one, check back on the Phone Update app until your phone reports that it is up to date.
More information on Windows Phone 8.1