Your old treads and risers will be put on with either nails or screws. If nails, just set the nails to make the removal easy!
Once done measure your riser first
Also measure top to bottom
If they are out of square you will have to use a adjustable square and then transfer that to the tread or riser
Put a nail in the top and bottom being sure to nail into the stringers
Put a Bead of liquid nail on stringer and a little on the riser
Put a nail in the back
Put a nail in the front at each stringer being sure to hit the stringer