The Easy Cutter Serrated Blade is different to the blades that come with the kit. It has a carbide insert in the cutting area for long life and tiny teeth on the cutting edge to grip into slippery material.
To change the blade simply use a ball point pen to push in the clip on each side and remove. Installing the new blade you will need to ensure the tongue inside the blade is aligned with the flipper on the body of the cutter, then it should locate and clip in easily.