Although the “official” launch of iOS6 isn’t until Wednesday, you can install the developer GM build right now. Usually Apple verifies beta software updates using iTunes and the developer’s UDID, however, GM builds are typically open to unregistered “developers” too. Just to clarify, iOS6 GM is the final version Apple will be releasing to the public on Wednesday.
Here are the steps:
1) Back up your iDevice. Do a full iTunes sync or iCloud backup to make sure you don’t lose your data if something goes wrong.
2) Download the latest version of iTunes 10.7
3) Go to this website to download the version of iOS6 for your device.
4) Locate the .ipsw iPhone Software Update file and drag it somewhere you can find it (like your desktop). If you use Windows, you will need 7-Zip to open the file.
5) Connect your device to iTunes and under the “Summary” tab, locate the “Check for Update” button. Hold down the option key (Mac) or the shift key (Windows) and click the Update button. Browse for the .ipsw file you downloaded and wait for the update to finish.