Given the age of your listed vehicle (2001) if the lenses are yellowed and cloudy, you can polish them back to clear.
If the lenses look alright, make sure your headlights are getting enough voltage. Over the years, the connections can weaken or corrode, so making sure everything is tight and has a good connection will increase the brightness.
Finally, you could try replacing the bulbs themselves, even though they aren't burnt out. Some bulbs do get dimmer with age, especially the cheaper ones. Hope this helps.