You will need to check the fare conditions for your ticket... but unless you are on an expensive full-fare ticket it is highly unlikely that you will be able to change your reservation without incurring a fee, if at all. The cheapest tickets usually have a "no changes, no refunds" clause attached to it, and there is usually very little flexibility in it. It doesn't hurt to ask (I've heard of cases where people have been allowed onto earlier flights), but be prepared for a "no".