How do you change the blade on this knife? It looks simple enough but I can't figure it out!

When you open the blade all the way out you will see a notch built into the blade carriage is holding the blade in. Push that notch or section laterally(sideways) with your thumb. That will release the blade and allow you to pull it out. Hope this helps.

To remove the blade, slide the blade all the way out. At the top of the extended blade, there is a small metal area to press down. While pressing down, pull the blade out.