Make sure you have at least 5 hours in your layover and that leaving the airport isn't going to stress you out too much. You will have to return through security. This works best if you do not lave loads of hand baggage or can leave it in a secure place.
If it's not rush hour, take a taxi for a ride of about an hour each way. All taxi drivers should know how to get there. If you are worried about traffic, see Step 2.
Enjoy a chance to check out the public transport of Kuala Lumpur. Take the Express Rail Link from the airport to the Sentral Station in KL. It will take 28 minutes.
Transfer to the PUTRA light rail and get off at the KLCC station. It is not too far.
Exit the KLCC station and look up. You will find the towers quickly. If you have arrive very early you can get a ticket to the sky bridge. They are free, but limited in quantity. If it's early enough, you can get your ticket on the concourse level in the visitor center across from the gift shop.
Stay and enjoy what you like but give yourself enough time for the return trip to the airport.
