There are several places in cancun and most of the time they all have different exchange rates, this rates vary for a few cents only but before changing your money make sure to compare the rates between at least 3 of this places. You will find the option to exchange your money at the airport, your hotel, the banks and malls. Don't forget to bring your passport with you everywhere you go since this is an important requirement to exchange your money and can be used as an official ID while in Mexico. Visit this link to see more information about money.