Unscrew the blade holder from the carriage.
Take the blade holder out.
There is a pin at the end of the blade holder that you press in and the blade will pop out so you can grab it and pull it out. Think of it as sort of like a retractable pen. Then you just slide the new blade right in. There is a magnet that will hold it in place. Twist the serrated part to extend the blade out about the width of a credit card.