Changing Flight Penalty?

I have booked a flight with Contential Air about a month ago. It was
the cheapest type of tickets. I am to fly about a month fron now. I
see by checking the ailine's web site that the flight is way under
booked. Also, I have come to the conclusion that it would be better if
I flew an earlier flight. What type of penalty would I incur if I were
to change the flight? BTW the flight has two legs and I would like to
change both.