Following this morning's announcement aboutUpdate 3, Microsoft has made the RTM version of the update available publically through its developer programs. But Update 3 can be installed by anyone who wants it now. So here's a guide to the update process.
Be sure to check out my post Windows Phone 8 Update 3 Preview for a complete rundown of what's new in Windows Phone 8 Update 3.
First, some important caveats
Don't even consider installing Update 3 until you've considered these issues.
Remember, Windows Phone 8 Update 3 is primarily aimed at new 5- to 6-inch "phablet" devices with 1080p screens. So check out the full feature list in my Windows Phone 8 Update 3 Preview post before even considering installing this update.
Update 3 requires that your handset already be updated to GDR2, the previous update. That's most phones, of course, but it's good to know.
Installing Update 3 may void your wireless carrier warranty. If you have a brand new phone, that's something to consider before diving in.
You cannot go back. Once you install Update 3 in this fashion, you cannot return to GDR2 or previous Windows Phone versions. You will later be able to update to the general release of Update 3, which will probably include firmware and driver updates too, or later updates.
Finally, the version of Update 3 described here includes only Microsoft's code. It does not include any device firmware or driver updates that may accompany the future, public release of his update.
Sign up as a developer
Technically, Update 3 is only available to those who have registered their Microsoft account—the same one you've signed in with on the phone—with one of Microsoft's developer programs. These include the Windows Phone Dev Center ($19 per year) or the free App Studio, which lets you create simple Windows Phone apps on the web. Since App Studio is free, head over there first, sign in with the same Microsoft account you used to sign in on your Windows Phone handset, and then register. That's all you have to do.
Download the Preview for Developers app
Now, download the Preview for Developers app from the Windows Phone Store to your handset. You can do this remotely, from your PC's web browser, or directly from the browser on your phone.
Run Preview for Developers
Once the app is installed, find it on your phone and run it. The app will step through a short wizard that explains what's happening, provides a Terms and Conditions document you must agree to, and a sign-in for your Microsoft account. Once that's all done, you agree to enable the Preview for Developers and click Done to complete the process.
Check for updates
Now, navigate on your phone to Settings, Phone Update and tap the Check for Updates button to begin the process of installing Update 3.
Download and install Update 3
Phone Update will automatically find Update 3 and begin downloading the update. When that's done, it will prompt you to install the update now. This requires a reboot.
During the update process, you will see the familiar "install gears" screen on the device, which indicates that something wonderful is happening.
This process should take 10 minutes or so, and after a second reboot, Windows Phone will migrate your data back much as what happens when you upgrade from one version of desktop Windows to another.
When that's complete, you'll be told that the update is all done and you can return to the Start screen.
You can ensure that the update installed correctly by navigating to Settings, About, tapping the More button and checking the phone version number: It should now read 8.0.10512.142.
Should you do this?
If you have to ask, probably not. But if you like living on the edge—and, to be fair, Update 3 is done, this isn't a beta version—and simply can't stand not waiting, it's proven to be reliable and error free for me so far. But understand that any phone update process like this certainly comes with some risk.