The airport has an exchange kiosk, although the exchange rate is not good. There is a yellow 5B ATM machine downstairs, on the left.
Good hotels in Zone 10 have reliable ATM machines, as does the new Walmart in Guatemala City.
There are two reliable ATM machines in Santiago Atitlan and three banks. Traveler’s checks can be changed in the local banks. Dollars and Euros are more difficult to change.