Assuming you would like tips on how to open the back casing of your HTC One:
1. With the phone facing screen-up, locate the small notch at the top-left corner of the phone
2. Insert a fingernail or other thin object into the notch and gently pry open the back casing
3. Be careful not to apply too much pressure, as this may damage the phone's internals
4. Once the back casing is pried open slightly, it can be slid off completely