You You do not necessarily need a new deck if a few steps are rotten.  it just depends on which ones.

Note from Les:  In this case they had two rotten steps.  The one at the bottom was very easy to repair but the one at the top was not.  6 boards were rotten including the frame which was holding the top step.  Instead of taking the while thing apart, I rebuilt it from the inside.  it took more time to do it, but it customer the customer much less than the alternative.