Before going ahead with the toner cartridge change verify you only need to replace the toner cartridge and not both cartridge and drum unit
Follow the steps below in order to replace the Toner Cartridge.
Ensure that the machine is turned on.
Open the front cover.
Remove the toner cartridge and drum unit assembly from the machine.
Push down the green lock lever and take the toner cartridge out of the drum unit.
Unpack the new toner cartridge and remove the protective cover.
Put the new Brother DCP-L2500d toner cartridge firmly into the drum unit until you hear it lock into place.
Make sure you put the toner cartridge in correctly or it may separate from the drum unit.
Clean the corona wire inside the drum unit by gently sliding the green tab from left to right and right to left several times.
Be sure to return the tab to the Home position . The arrow on the tab must be aligned with the arrow on the drum unit. If it is not, printed pages may have a vertical stripe.
Install the toner cartridge and drum unit assembly back into the machine.
Close the front cover of the machine.
After replacing a toner cartridge, DO NOT turn off the machine or open the Front cover until the machine's display returns to Ready Mode.