On the bottom of the drone, find the black cable shown in the red circle. If you're removing the front right arm: look for the black cable. If you're removing the front left arm: look for the white cable. Both cables are relatively close to each other.

Once you've found the cable corresponding to the arm you're replacing, remove it. I find it easiest to use needle-nose pliers and pull straight upwards on the connector. The cable should come off easily.