There is patch of dark brown skin. Around my wrist what could it be?

Send us image. Can you send us an image of it? Without seeing it, it's impossible to tell what it is.
Wrist patch. Brown spots can because by many different things. One of those is sun damage but at 26 i would not expect that to happen. It could be that you spilled some lime juice on your arm and then got in the sun! i named a condition called margarita nose in which this happened as a transverse line across the nose which was hyperpigmented or darkened.