Replacing the drum unit on a Brother printer is quite straightforward. All you need to do is to follow the instructions given below:
- Keep the printer turned on, and open its front cover by pressing the release button.
- Hold the drum handle and slightly slide the assembly towards you. Then, place it on the paper.
- Press the green lock lever on the side of the drum unit. Then, pull the toner cartridge upward and out. Now set the toner aside for reuse in the new drum.
- Unpack your new drum and protect it from direct sunlight. Remove all packaging materials such as shipping labels and plastic films.
- Insert the toner cartridge into the new drum until you hear a clicking sound, indicating it is in place. Once it is seated correctly, the green lever will lift back on its own.
- Clean the corona wire, which is a small tab labeled with an arrow or marked in another color. Slide that tab back and forth 5-6 times to clean the wire, and return the tab to its original position.
- Reinstall the assembly by sliding the drum back into the printer using the guide tracks until it stops. Avoid closing the front cover, as the counter reset requires the cover to be open on most models.
Source: How to Replace the Drum Unit on a Brother Printer