DIAGNOSIS: It's normal for these routers to feel quite warm, even hot, during operation. They have no internal fan and rely on passive cooling through those vents on the top and sides.
FIX: Your main job is to ensure it can breathe. 1) Check its location. It should not be in an enclosed cabinet, on top of another hot device like a cable modem or amplifier, or buried under papers. 2) Make sure all the vents on the top and sides are completely unobstructed. Give it at least a few inches of clearance on all sides. 3) Stand it upright if it's lying flat. The vertical position, with the feet on the bottom, promotes better airflow through the chassis.
IF THAT DIDN'T WORK: If it feels hot enough that you can't keep your hand on it comfortably for more than 5-10 seconds, or if you notice performance issues (dropping connections, rebooting itself), it might be overheating. Try pointing a small desk fan in its general direction to see if the temperature and stability improve.
HEADS UP: Avoid aftermarket 'cooling pads' designed for laptops. They can interfere with the router's own designed airflow and sometimes even introduce electrical interference that hurts your WiFi signal. Good old-fashioned space and airflow is the best medicine.