Do you mean earlier the same day or an earlier day of departure?

If the former, just go to the airport and ask to be put on an earlier flight. There is no fee for this, but there has to be room, so if the flights are full/oversold, then you may have to wait until your original flight time. If you want to be sure to be on an earlier flight, then you would have to pay the 100 bucks to be confirmed on that flight.

In the latter case, it depends on your booking class, and the rules associated with that class. Most tickets will require a fee, but in some markets, even the lower fare classes are refundable/changeable. I recently bought a IAH-MSY booked in S class that was fully refundable/changeable.

K