How to install iOS 6 before September 19

Apple has finalized the iOS 6 software that it first previewed months ago. Developers have had access to some builds to test apps, but now there is a relatively bug-free Gold Master build out there that anyone can use. This is the same file that is going to be pushed to users through iTunes on September 19th. If you’re anxious to see what’s new, why wait?
Before we get started, make sure you back up your device through iTunes. Most of your content should be in iCloud, but better safe than sorry. You’re going to be installing directly from Apple’s Gold Master of iOS 6, but there is always some risk involved. Proceed at your own risk. On the upside, you get the software now, and can avoid all the congestion next week when everyone else destroys Apple’s servers trying to get iOS 6.

1) Download the correct update file

Apple’s devices are not quite the monolithic block they once were. There are slightly different versions of several devices, and you need to know which one you have. For example, there are GSM and CDMA variants of the iPhone 4, and each one has its own update file. Even more confusingly, there are two hardware revisions of the iPad 2 (one had an updated ARM chip). You also have to worry about the presence of 3G/4G radios in the various iPads.
Here is a page with links to all the update files, both direct download and torrents. The direct downloads are fairly slow and the file is nearly 1GB for some devices. The torrent might be a better bet (there are plenty of seeders).

2) Extract and verify the update

You will end up with a .dmg or .ipsw file after the download has completed. If you have a .dmg, Mac users can just open this and extract the necessary .ipsw file. On Windows, you need an archive manager like 7-Zip to open the .dmg. Windows users will also have to then dig into the .hfs to find the .ipsw file.
Put this file someplace you’ll be able to easily locate. You should also check to make sure it’s about the right size. Your update file should be between 850MB and 1.1GB for most devices. The iPhone 3GS is a bit smaller, at around 780MB.

3) Update iTunes and install

Get the most recent version of iTunes from Apple (currently 10.7) and install. When that’s done, plug in your iDevice and make sure it is detected by the software. This is a good time to make sure that you’re completely backed up. There’s no going back after this, and you don’t want to rely completely on iCloud.

On the iTunes device screen, you need to select the file extracted earlier as the restore source. On a Mac, hold the option key and click ‘Restore.’ On a Windows machine, do the same thing, but hold Shift. This will bring up a file browser so you can select the .ipsw file. The update will take a few minutes — just don’t unplug your device for any reason.
When everything is finished, you can restore from iTunes or iCloud if necessary. Enjoy iOS 6 on your device before the riffraff gets it.