On the top of the drone, you should notice the 12 wires shown in the 1st image. These wires control the front right & left arms. You will need to de-solder the wires corresponding to the arm you are replacing.

The red line shown in the image separates the wires into two groups. The wires on the left, power the left arm; the wires on the right, power the right arm.

Once you de-solder the wires (and assuming you've disconnected the antenna cable), the arm should be completely removable.