iOS 6 is now available for everyone to download. Most users are best served updating to iOS 6 through OTA or iTunes, but advanced users can upgrade manually with firmware files. This has several advantages, particularly for those upgrading multiple devices in a household or institution, or for those updating from older versions of iOS.
The following firmware links are hosted directly by Apple, using IPSW files requires iTunes. You will want to have the latest version of iTunes installed before attempting a direct firmware update with IPSW.