In a gentle counter-clockwise move, you must twist the back end where the clip is located and align the solid dot mark from the back end with the empty dot mark from the barrel of the pen. This way you will reveal the battery compartment. Once the battery is installed/replaced, you will need follow the opposite steps to finish the proper installation of the battery.
Warning: Do not pull or twist the button located in the back end of the pen. If you do, you may cause damage to it and this may affect the functionality and integrity of the pen.