Change date of flight from Beijing to the London Heathrow via Moscow.

Next month I am flying from Beijing to the London Heathrow via Moscow, but need to change the date. What is the easiest way to do this with Aeroflot, and where can I find the fee for changing? Reading internet sites, some people say it's not possible to change with this company in Economy, and no-one answers the phone. Should I try to contact the company directly or the agency I bought the flight with?
Thank you.