Repairing Broken Stair Tread
You should be able to lift the tread and shine a flashlight in there and see that the back side of the tread that is mortised into the stringer will have a wedge drove under it. The mortise is made with a slight taper and when they install the stairs they work from the top down and install the tread into the mortise and then drive the wedge in to make a tight joint. Usually some glue and toenailed finish nails are also installed which will make for a tough removal. The only advise that I can give is to carefully pry up on the tread from the balluster end and try to pop it loose from the mortise but this may break something if not done slowly and carefully.
Hope this helps and you have success, Dave.
Do one thing at a time, do it well, then move on.
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