I have this dark patch (covers length corner lip to corner lip)in my mentolabial sulcus for couple years now. i just want to know how this is treated?

My understanding: is this is on the inside of your mouth where it doesn't show to others. I recommend that you see an oral surgeon and get a diagnosis. If it's nothing to worry about, then the best course of action is no action at all as long as it doesn't interfere with oral function. But get the opinion of a qualified oral health specialist first. Dr. Anne/