You can exchange money at the banks and ATMs. Don’t ever change on the street with money changers (people who offer to give you a good exchange rate); it’s just not safe. You’ll get a fair exchange rate at any bank or ATM. Please plan around the bank’s schedule, which is more or less from 8:00 am to 4:00 pm. Also a lot of ATMs across the country are closed between 10:00 pm and 6:00 am for safety reasons. Costa Rica’s official currency is the colón, but U.S. dollars are widely accepted, normally at a set exchange rate of 500 colones per dollar, although grocery stores and larger shops will respect the exact rate for the day. Just bear in mind that $100 U.S.D. bills are usually not accepted, so carry around smaller bills to make things easier.
Don’t exchange money at the airport, as the rate is not convenient there. Just exchange as little as you must there to get you through the rest of the day and exchange the rest when you reach a proper bank.