Exchange rates never change. Always in flux. Just assume that's a reality you can do nothing about and move on.
You do have options...
Now different places will have different exchange rates, you have the kiosk at the airport (generally bad) and you have banks and places like AMX (generally good) or you can take your ATM to a bank and withdraw currency in the country. Some countries will even accept US duckets and exchange them back you so that is also an option. Also, there are apps that show current exchange rates you can use.