If you are travelling in a spanish-speaking county and you want to change your money you must say:

-Donde puedo cambiar mi dinero?(Where can I change my money?)

In the exchange office you need to know what's the exchange rate then you should ask:

-A como esta el cambio?(What is the exchange rate?)

If you need to ask the cashier for coins or bills you must then say:

-Necesito cambio pequeño(I need small change)
-Necesito billetes grandes(I need big bills)
-Necesito monedas(I need coins)