Flight changes

So here's a question: if I want to changeĀ  a flight, I have to pay often outrageous fees to do so. But for the airline to change my flight, it just takes an email. This happened to me with Aeromexico, aka, Aeromaybe. I had a 2 pm flight from Mexico City to Guadalajara which they cancelled. Now I have to spend seven hours in the airport in DF cooling my heels. Where can I send my change of flight charge???