Look up under the porch steps to determine where the risers are positioned. You'll need to know where the risers are to avoid cutting into them -- you might be able to see them in the gaps between steps.
Put on safety glasses. Position a circular saw blade on top of the step, inside the riser. Cut into the damaged step on the porch with a circular saw, cutting parallel with the length of the step until you reach the riser on the opposite side.
Wedge a claw bar into the cut line and pry upward to loosen the step. It might come free, or you may have to pry at several different points along the cut. The step should snap in two, exposing the risers and nails.
Pry the other half of the porch step up with the claw bar then remove any nails remaining in the porch with a nail puller.
Insert a step plank of the same dimensions as the old step into the empty space. Fasten it into place with a hammer and stainless steel nails.
Seal the new porch step with wood sealer and a brush. Let dry, as directed by the manufacturer, before using the new porch step.