You will need a long, thin tool. A hex key, a VEX shaft, or even a paper clip can work.
The reset button is a white button that is located at the bottom of the hole closest to the center of the controller.
You can confirm that this is the correct hole by using a flashlight and peering into the hole. You should see a white button, not a screw.
Use the tool from the first step to press and hold the reset button for 5 seconds.
Then release the button and power the controller back on.