Installing treads and risers to existing stair runs
The stair treads are set to a base material, if you are lucky it is not glued down. Just pull them off and removed the nails or screws that are left over. 

You will most likely need to do something with the base material as well. The thickness of the new material most likely will be thicker than the cap material you remove, making the first tread from the lower level as much as 1/2" different than the rest, creating a trip hazard. This will occur at the top too.

Code allows a difference of 3/8" total from the largest to the smallest tread.

A picture would really help detail the answer better...