The last question is, of course, the most pertinent. The entire point of canceling an extended warranty, which is a service contract, is to get your money back. After all, why would anyone want to cancel something they already paid for if they are not going to see that money again?
The answer, unfortunately, is buried in the fine print of the warranty contract. If you did not pay attention to it beforehand, you probably want to dig out your copy of the extended warranty and take a good look.
Extended warranty refunds are pro-rated on the amount of time/miles the warranty was in force. There will certainly be a cancellation fee. Some service contracts will offer a full refund if you cancel within a certain time period after signing, usually30 days. The worst case scenario is the non-refundable extended warranty. Yes, they exist and usually are not sold by any reputable company. This is why it is doubly important you make sure to research the company you are buying a warranty from.
To cancel, you will probably have to fill out a cancellation form with the dealership or manufacturer behind the warranty. Even if you purchased a warranty from a third party company, the warranty may actually be backed by a dealer and your cancellation request will have to go through them.
A request for an odometer reading is standard. Check your warranty company to see if they will accept a third party odometer reading from the last time you had your car serviced.
Having provided all the proper documentation, be sure to request a dated hardcopy confirmation of the warranty's cancellation. Whatever agents or middlemen who sold you the warranty most likely received commission for it; by requesting a refund, they are likely to lose all or some of that commission in the process. Because of this, they have every incentive to sit on your cancellation request for as long as possible. Verbal confirmation is insufficient; pester them until you receive a dated copy confirming your cancellation.
Once the cancellation request has been processed, it is fairly typical for the refund to take several months.