Rip out any old or rotten wood. Use a prybar and place the end underneath the step, between the supports. Pull upward to loosen the board. Keep prying until it comes up.
Check the stair supports. If this wood also has rotted, it will need to be replaced. If it is simply old but still solid, you can use the supports.
Cut the new boards to fit. Purchase pressure-treated wood from a lumber store. This type of wood will hold up to the elements and last much longer than untreated wood; however, if you are only replacing one step, purchase the same wood used on the porch before, if available. Use the old boards as a template to measure the length and width. Use a circular saw and cut the new wood to fit.
Install the new stairs. Do not nail the stairs in; this will only cause problems in the future. Use wood screws that are an inch longer than the thickness of the new boards. If the supports are located underneath the step, insert the screws through the boards, into the supports, using a cordless screwdriver. If the supports are on the side of the step, insert the screw on the outside of the support and run it through the edge of the stair board.
Seal the wood. You can stain the new wood if you would like, or just cover it with a coat of clear exterior wood sealant. Sand the new wood with a fine grade of sandpaper and wipe off any dust. Apply the sealant or the stain and then sealant with a paint brush. Let this dry for six to eight hours before allowing anyone to walk on the steps. If you will be painting the stairs, prime them first with an oil-based primer. Let this dry, sand it lightly and then cover it with an exterior oil-based paint.