How can I increase melanin?

Not really possible. Your ability to make melanin is genetically predetermined. You can't make more of something that you aren't naturally going to make. Not sure what you are hoping to accomplish but i hope that helps clarify a bit for you.
Can't . It is just your body. Going out in the sun it brings out your melanin but that is bad by causing skin cancer and premature aging.

Related Questions

I have a pink lio scar and my lips are naturally brown. How can I increase the melanin in lips to even out skin color?

Plastic surgery. Seek consultation with plastic surgery regarding tinting your lip color. . Read more...
See a Dermatologist. You should visit a dermatologist that specializes in the treatment of lip pigmentation and skin texture. Read more...