A photo will help with providing proper direction. But I'll provide general direction that may or may not help. Usually when I have removed treads I make a few cuts with a circular saw being careful that the blade depth is set to exactly the thickness of the tread, then with a wood chiesel that I keep for this type of situation I start splitting pieces of the tread out, finishing up carefully with a reciprocating saw. The riser is likely screwed or nailed into the back of the tread, the sides of the tread set into the stringer. Once the old tread is out you can determine the length to cut the new tread and also, if necessary, to rip the new tread, and best way to install it. Be sure to secure the new tread well or you will have a squeak every time someone steps on it. post the photo if you can and good luck.