Are you sure of the error message? The Brother printers that I have used have Drum Ends Soon and Change Belt messages.
The Brother MFC-9440CN has a belt that can be replaced. The Belt unit has an life expectancy of ~50000 pages. The belt unit is part BU100CL and costs a bit over $150 at office supply stores. Amazon.com and some other stores have the unit for under $100. http://www.amazon.com/Brother-BU-100CL-HL-4040CN-HL-4070CDW-Series/dp/B000SC245E
is one source.
When the unit says Change Belt Unit, follow the directions from the manual's maintenance section to replace the belt unit. See page 178 (page 189 of the PDF) http://welcome.solutions.brother.com/BSC/public/files/dlf/doc002252/mfc9440_usa_usr_b.pdf
. You have to remove the Drum and then the Belt Unit. Then slide in the new Belt Unit and replace the Drum unit. Then close the printer and turn it on. Finally reset the counter from the Machine Info sub menu (press Menu 8 4 and select the Belt Unit and press OK. Then press 1 followed by Stop/Exit).
If you mean the Drum Unit, that is part DR-110CL and costs over $175 at the office supply stores. Order this part when the Drum End Soon message appears and replace it if the print quality deteriorates (or when you have the new drum). The Drum Life is ~17000 pages. The steps start on page 166 (182 of the PDF). You need to remove the old Drum unit. Then remove the toner cartridges from the drum. Prepare the new Drum Unit, including putting the cartridges in the new Drum. Then replace the Drum unit and reset the counter.
I hope this helps.