To replace ink or toner cartridges in your Brother printer, follow these steps. Start by turning off the printer and opening the front cover. Gently grasp the old cartridge and carefully pull it out. Unpack the new cartridge and remove any protective seals. Slide the new cartridge into the designated slot until it clicks securely in place. Turn on the printer and shut the front lid. It will automatically recognize the new cartridge. If prompted, run a test print to ensure proper installation.