Imaging drum is different from toner cartridge. Your printer use toner cartridges for each color and one drum unit that is shared by all colors. Both toner and drum are consumables and your printer will ask you to replace as they are used up.
Drum is one of the supplies inside your machine that can be pulled out and replaced. it is just above the paper feeder. You have to life up the cover and levers to remove it. If you have not done this before, please see user guide on replacement procedure.