Use a concrete saw with a hose attachment to remove damaged part of step. The hose helps keep the blade cool and keep down dust. ( Follow the manufacturer's instructions for safe use of the concrete saw.)
Build a form for the step. In our case, we used a piece of lumber. Spray form with cooking oil to keep it from sticking to concrete.
Mix concrete to manufacturer's instructions and pour into form. Tap form to remove air pockets in the cement and to help it settle. Use a magnesium float to smooth the cement. Finish off with a trowel. Remove form. Avoid walking on the step for 24 hours or more, depending on the cure rate for your cement product.