The Casio Pathfinder's watch band is attached to the watch face like most other watches. The small pin that connects the band to the face is known as a spring bar. The spring bar is simple to access as long as you have the right tool. A spring bar tool can be purchased online or from any jewelry or watch repair shop. If you don't have one, you can use a flat-head screwdriver. Unless the tip is very small, however, you'll have trouble releasing the spring bar.
Bring the Casio watch to a table. Position a cloth or towel on the table, and then place the watch face down on the cloth. This prevents the face from getting scratched.
Push the edge of the band that attaches to the face to either side. This creates a bit of working room between the band and the mount, allowing you to access the spring bar.
Insert your spring bar tool in between the band and the mount, onto the edge of the metal spring bar. Pull the spring bar inward with the tool and simultaneously push downward. The spring bar will disconnect, freeing one edge of the watch band.
Slide the other edge of the watch band away from the mount.
Repeat the process for the other piece of the watch band. If your replacement watch bands did not come with their own spring bars, slide the spring bars out of the old band and insert them into the new ones.
Insert one edge of the new watch band into the mount. The spring bar on the other edge will rest on top of the mount on the other side.
Push the exposed spring bar inward and simultaneously push the watch band downward. Pull back the spring bar tool. The spring bar should snap into the mount.
Repeat the process for the other piece of the watch band.