Okay, so I've got to ask you to consider why you think your lips need lightened? We as human beings are so much more than the totality of our physical features, and yet we place so much emphasis on those. That being said, the first best place to start is using a lip balm or gloss that has a high SPF each and every time you go out.
Hyperpigmentation is often caused or worsened by exposure to the sun. It is recommended that when in the sun you cover the area with a SPF 30 sun screen protectant. There are some makeups that have sun protection in them, not sure of the SPF. Some of the treatments for this are skin bleaches and botanical skin lighteners. A common prescription used is Hydroquinone.
Are you asking how to do this?
A lot of people use a "skin lightener" that works by making your skin stop producing so much melanin. "Excessive" melanin production is one of the causes of dark lips and the use of this lightener would reduce this.
If you want more information, use this link:
First, check out what is causing the darkening of your lips.
If you have not a dark skin complexion, maybe your lips are getting darker doing something.
Maybe you are drinking a lot of coffee and you are smoking a lot, or maybe you are using B standard lipsticks.
A good home remedy that helps is to apply lime juice or apple cider vinegar on your lips. Also, exfoliating the lips with lips scrub could also help to remove the layers of dead cells and to reveal new layers of cells.
But before doing something to get your lips lighter, quit the habit supposed to make your lips darker and darker.