Is it possible to increase melanin or change eye colour from green to brown?

No: No. There is no elective medication to change eye color. In individuals affected by the disease glaucoma, a therapeutic agent used to lower eye pressure, called a prostaglandin analogue, has a side effect of changing iris color. If you need glasses to see well, you could discuss with an ophthalmologist wether contact lenses are right for you. These can be colored.

Side affect: The glaucoma eyedrop Lumigan directly applied to eyes, could change their color. Latisse, (bimatoprost) for lengthening eyelashes, has the same warning because it has the same chemical composition.But you do not apply latiesse to the eyes. This can happen only if you have hazel eyes, or blue/brown or green/brown eyes. Pure blue and green eyes don't have enough melanocytes to get darker.

Not possible: The internal eye color from the iris is permanent. There is now way to change this with medication or surgery. If you want a different eye color, you can check with your ophthalmologist about colored contact lenses.