Brother HL-5450DNT - Manual (Page 114)

108
Routine Maintenance
5
f
Put the toner cartridge firmly into the new drum unit until you hear it lock into place. If you put the cartridge
in correctly, the green lock lever on the drum unit will lift automatically.
Note
Make sure that you put in the toner cartridge properly or it may separate from the drum unit.
g
Put the drum unit and toner cartridge assembly back into the machine.