As a general rule, when you sign a contract to buy something you are bound by the terms of that contract. There is no time limit within which you may just "change your mind." In most cases, to get out of a contract, you must show that you were induced into the contract by fraud, duress, deceit or misrepresentation. There are a few exceptions to this rule.
It is possible that your contract or purchase agreement contains a cancellation clause or a return policy. You should review the terms of the agreement you signed to see if it contains any information about cancelling the purchase. Only an attorney can provide you with a legal opinion or advise you of your rights in a situation.