Place the device on a flat surface (with the headphone outlet facing you). Using one of your fingers, place the tip of your finger over the highlighted spot (as shown in the second picture). Next, pull the back cover upwards.
If the first method is too difficult, use the plastic opening tool in order to complete this step. Stick the plastic opening tool inside of the gap. (The gap is located on the top of the device which is highlighted in the second photo). Using the plastic opening tool pull the back cover upwards until the back cover pops off.