You can't. You can put it in it's USB charger and put a paperclip through the hole in the back and hold it there for 3-5 seconds. However, there is no way to reset it to factory settings and start over. If it is acting up, as mine did showing 19,365 steps 20 minutes after I got up, you will most likely need to replace it.
i don't know what the Fitbit policy is, but they replaced mine after it started false-reporting 5 months after I bought it.