Microsoft has officially announced Windows Phone 8.1, but is yet to release it to the general public. That said, the developer preview of Windows Phone 8.1 will be made available to registered developers soon and developers will be the first to get their hands on the final RTM version of the mobile OS once it is released by Microsoft.
If you want to try your hands onto the developer preview of Windows Phone 8.1 and want to be the first one to get the finished version of Windows Phone 8.1 when it does get released, here are the steps you need to follow.
Register as a developer
You need to first register as a developer with the Microsoft developer program. This process is free and you will not be required to pay a single cent / penny for it. Also you need not be a developer to actually register as a developer with Microsoft.
The trick is to register using the free App Studio that Microsoft released recently. Use the same email address as you have used on your Windows Phone handset and register as a developer.
Download Preview for Developers app
Preview for Developers app is basically an app that configures your phone to receive developer previews of software updates that are yet to be released by Microsoft.
Download the app from this location and install it on your Windows Phone handset.
Once installed, run the app and agree to the Terms and Conditions, sign-in using the Microsoft account using which you registered as a developer. Then agree to enable Preview for Developers and click done to complete the process.
Check for phone updates
Navigate to Settings -> Phone Update and tap on check for updates. This will connect through to Microsoft servers and check for Windows Phone 8.1 developer preview.
Your Windows Phone handset should find the developer preview update once available and begin downloading it.