I downloaded IOS6 on my iPhone 4S. It appeared to have done the download, with the progress bar completing the first step. It then froze midway into the second step and will do nothing
IOS6 on iPhone: Frozen while updating?
Was this by w ay of OTA or with a computer? Just go ahead and try it again with the computer, connect the iPhone, open iTune and let it update.
If it doesn't for any reason, then download IOS6 and save it in your computer, and when you open iTune, press and hold the Shift key (if on Windows) and click restore at the same time, from here search the saved firmware and start with the restore process.
Best of luck.