It's the round circular multi directional button below your right thumb on the back of the camera. Though you may have it locked .. there is another button on the back surrounding the multi directional button that unlocks the focus selector.
Use your thumb on the Multi-Selector on the back of the camera to choose the Active AF point. Make sure that the locking ring around the Multi-Selector button is set to the dot and not in the "L" locked position. Also make sure the camera is set in Single Point or Dynamic Area AF mode via the AF Area mode switch below the Multi-Selector on the back of the camera. Good Luck and Enjoy your Nikons!