Warning: Blades are sharp and can cause serious injury. Be extremely careful when handling blades. Wear eye protection and dispose of used blades properly, such as wrapping in duct tape.

FULLY EXTEND THE BLADE.
Press down and forward again on the slide button until the button is in the full front of the top slot (0 through 3 in the below picture).

Note- the blade must be extended beyond the 3rd detent position. This will cause the slide button to remain in a down position, allowing the blade release button to operate.

PRESS AND HOLD THE BLADE RELEASE BUTTON ON THE FRONT SIDE OF THE KNIFE. (Step 4 in the below picture)

Note- if the blade is not fully extended beyond position 3) , the blade release button will not work.

Remove the old blade carefully.
Insert new blade.

NOW, RELEASE THE BLADE RELEASE BUTTON.
ENSURE THE BLADE IS LOCKED IN PLACE. If the blade does not retract, try carefully giving a slight tug on the blade to reseat.