Hi Mansi89,
Over the last several months, there were no fees involved in rescheduling or cancelling an appointment for the Online/At-Home GMAT. However, GMAC recently changed its policies (and now there are fee associated with each of those options). The link that catinabox provided explains those fees in more detail.
As such, if you don't want to keep your scheduled appointment, then you're likely going to be 'stuck' with a fee of some type. That having been said, if you reach out to GMAC's Customer Service and explain that when you made the appointment, it was under the assumption that there would be no cancellation fee, then you might be able to talk the Customer Service Team into removing the fee. For practical purposes, I can't say how likely your chances are in this regard - but it would be a reasonable argument to make (and one that GMAC might be open to).
GMAT assassins aren't born, they're made,
Rich