Not a simple task, you have to tear out the old stairs, including the stringers and make new stringers. It may not be practical or even possible depending on space availablilty.
Typical stairs by todays code should total about 18" in rise and run. IRC R311.7.4 deals with treads/risers. Max riser height is 7 3/4", min tread depth is 10". There are other issues such as, available headroom, that might limit what you can do.