LEDs by their very nature last for thousands of hours of lighting, so you should not need to replace an LED puck light very often. But if you feel the brightness is being lost over time then it is not a hard job to replace the LED bulbs in some models.
You will need to turn the power off and remove the cover from the fitting, then it is just a case of replacing the bulb. Obviously all puck lights will vary so it is best to check the manufacturers' instructions to not only ensure correct replacement but also safety measures required.