What steps are required to replace a toner cartridge in the HP LaserJet Pro MFP M521?
To replace a toner cartridge, press and open the front door. Remove the old cartridge from the printer and place it in a bag for recycling. Insert the new cartridge by aligning it with the tracks inside the product and firmly seating it.
From the official HP Laserjet Pro Mfp 3101 3108fdn Fdw Printer manual, page 36
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