How to put another country in a GPS?

well i got a Curtis GPS system the model is GPD 430 and im going to go to mexico in a month nd stay there for a while, and well im going to need a GPS and i have the curtis but it doesnt have the country Mexico in it, so i was wondering if theres a way to put the country map in it to be able to use and not buy a new one, if there is then how do i do it? thanks!!