DIAGNOSIS You're likely pulling from the wrong spot or not applying enough pressure to the release mechanism. The cover latches securely at the top.
FIX Place both hands on the sides of the purifier. Press firmly inward on the two indented areas at the TOP of the rear cover with your thumbs. While maintaining that pressure, pull the cover directly towards you. It should pivot open from the bottom.
IF THAT DIDN'T WORK Check for an obstruction. Ensure the unit is on a flat surface and not being pushed against a wall. Look at the bottom hinge area to see if a carpet fiber or debris is jammed in the gap. Clear it and try again with the press-and-pull motion.
HEADS UP Never use tools or excessive force to pry the cover off. You risk breaking the plastic latches or hinge. If the latches are genuinely broken, the cover won't stay sealed, ruining the air seal and making the unit ineffective.