It used to be in the storage menu, on my Gnex there's an option under backup and reset. However to me the best option is in stock recovery. Turn your phone off(or battery pull) turn it back on holding power and volume down. Select recovery using the volume up/down key and then click power. In the recovery menu choose factory reset and it will handle the rest.
Just wanted to thank you Mr Smith, and chime in with my experience. IF you are having issues with your Rezound after the ICS, do a hard reset. DO IT! Since updating my Rezound, performance had been bad. Everything worked just well enough for me to procrastinate in dealing with it, but it was really slow and laggy, and worst of all, music would cut out momentarily while playing once every minute or two, which was driving me nuts! I hadn't been able to find alot of chatter online about performance issues w/the ICS upgrade. I even thought about either rooting or somehow rolling back to GB, neither of which I wanted to try to do.
Googled the issue yet again the other day and came across this thread. Rashly decided to do the factory reset then and there. Pulled battery, held volume down button while powering the phone up. Came to a DOS-like menu, chose factory reset, hit power to select.
It worked! Of course, all my apps and settings were gone, but it did NOT erase the SD card as others have alluded to. And the performance...oh my god it is sooo much more smooth and responsive. No more music cutting out. And the battery life seems to be improved now that I've cycled/recalibrated it. After nearly 9 hours today, I'm still at 75%, with screen at 25%, mostly on wifi/ a little 3G & moderate use including a few short phone calls, 40 minutes or so of music playing, and some web surfing/texting/playing.
So for anyone like me who was dismayed by their Rezound's performance after ICS update, do the factory reset!!!!