Brother MFC-8420 User Manual page 407

Follow these steps to replace the drum unit:
1
Press the cover release button and then open the front cover.
2
Remove the old drum unit and toner cartridge assembly. Place
it on a drop cloth or disposable paper to avoid scattering toner.
Caution
To prevent damage to the printer from static electricity, do not touch
the electrodes shown below.
22 - 33 TROUBLESHOOTING AND ROUTINE MAINTENANCE