I bruised my knee 2 years ago but the bruise is still there and it's black. How do I get rid of it?

?? If it is 2 years old it is unkikely to change much in the future although it may very gradually fade this is due to deposition of hemosideriin in the skin from the blood that was the bruise most bruises clear with time but not all sorry but I do not know of any treatment that might make much difference.
Need more info. It could be that the blood underneath the skin has tattooed the skin or some other esoteric reason. Consult with a dermatologist.