They probably will charge you a penalty to get out early, even if it's only a little early. The only way to know for sure whether there's a penalty, and how much, is to call them and ask. It's possible the penalty will be small, because they might have pro-rated penalties (this means that the less time there is left in your contract, the smaller the penalty gets).
There's a simple reason why they have a penalty - part of what you pay every month actually goes to finish paying for your phone, which you either got free or at a discount when you started the contract. If you leave the contract early, then you haven't finished paying them for the phone, so the penalty helps them get their money back (and of course, the penalty also helps prevent you from leaving in the first place, which also is good for their business).
There's a simple reason why they have a penalty - part of what you pay every month actually goes to finish paying for your phone, which you either got free or at a discount when you started the contract. If you leave the contract early, then you haven't finished paying them for the phone, so the penalty helps them get their money back (and of course, the penalty also helps prevent you from leaving in the first place, which also is good for their business).