@Leah Brooke Depends on the outcome you are looking for. The work would be done by a concrete contractor if you demo the top concrete slab and install new. There are also concrete repair businesses that might have a product and service to sell that would me more cost effective. They would likely apply some high grade epoxy.
The crack is not of huge concern to me. The deteriorated and exposed aggregate on the front would be of concern to me. It's likely that somewhere in the past someone used salt on the step. This salt reacts chemically with the cement in the concrete and weakens the bonds with the sand and rock and you see the concrete begin to crumble.
It appears at some point the the concrete was painted to reduce the water infiltration and protect it from the freeze thaw cycle. It appears to have worked fairly well but the paint has started to break down. You could scrape of the loose and repaint and with a quality concrete paint it would likely last another 10 years if you are careful to keep the snow off.
Just guessing on cost, a new top slab would be most expensive ($1,000 - $2,000), the epoxy solution ($300-$500) and paint the cheapest if you DIY ($20-$50).
Try homeadvisor for some quick quotes. I'd have it replaced or at least ask for it.
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