Once the toner cartridge is empty, the printer will display a “replace toner’ message and you won’t be able to print further till the ink cartridge is replaced. Let’s see how to replace toner in Brother printer. Ensure that your printer is turned on. Open up the front cover of the machine. There are two slots, drum unit and toner cartridge assembly. Now, slowly push down the green lock lever and take the toner cartridge out of the drum unit. Put the new toner cartridge in to the drum unit after shaking it. If the installation was successful, the green lever would fit automatically.