I agree with Gary, the detail is in the contract and most reputable Realtor will allow you to break the contract if you explain the situation to them.
If you changed your mind about selling, which happens from time to time - although I always make sure my clients really want to sell before I list with them but situation can change - If this is the case, you can keep the house listed with the same agent, but just ask him/her to take it out of the market until the listing period expires.
or, I wonder if you want to break the contract because you are not happy with the performance of the agent? If so, have you discussed this with your agent? If not, please do first discuss with your agent about your expectation and why you are not happy with the performance. I am sure your agent will be very happy to have the feedback and perhaps change the approach to meet your expectation. If you have already done that, you might be able to talk to the broker also. I would think they want to do the right thing to serve you.
Again, discuss this with your agent and get an agreement with him/her. I think they will respect your wishes.