how i can buy luggage check in for China United Airlines?

I bought a flight from shanghai to beijing on Oct 25 without pay attention to no luggage check in. Now I'd like to change my ticket and/or additional fee to buy luggage check in for this flight but I couldn't find the website of China United Airlines. What should I do? Can I pay the luggage check in fee when do check in at the airport before depart? Or I should contact the airlines in advance to inform?