Only with that agents 'Brokers' permission, or if the agent did not perform theri duties.
If you have a valid listing contract, you are bound to that unless the agent did not fullfill their duties as written in the contract or the broker releases you from that contract. Some agencies will not cancel the contract under any circumstances, except under a court order.
We have a written guarantee that if a client is not happy with our services we will cancel the contract. After all, we provide a service to a client and that service should be top notch at all times.

If you are having problems with your current REALTOR and wish to switch, you should place a call to that agent's office broker. The contract you signed (I'm assuming it was a buyer agency agreement) is actually with the broker and not the agent. I'm sure the broker would welcome the opportunity to assign you to another agent within their office or company in order to retain your valuable business. Just because you don't "click" with your current agent does not mean you need to go to the extreme of switching companies. The broker knows the skill level and personalities of their agents best so give them a shot at matching you with someone else. Good luck.

Please review the fine print of your contract. There should not be any fees involved but there needs to be some form of notification that you want to cancel your listing and a form that needs to be signed by you.