If you can't wait, just open the Settings app, tap General, and then tap Software Update.
Be sure to back up your phone and keep an AC adapter handy.
But the conversion process -- which has to take every file on your i Phone, i Pad, or i Pod touch and convert it over to the new file system -- takes time.
But given the fact that if it goes wrong then you've lost your data, it's for the best it takes all the time it needs!
And the older your device, the slower the install process seems to be, with some devices taking over an hour to update. The answer is the new file system that Apple has introduced.