Easier and cheaper to just replace.
If they're crumbling, they may need to be replaced, Harold
Remove and replace!
Harold....If they are really bad you may have to be removed completely as repairs will probably not work or last
If they are concrete, brush on a bonding liquid first after brushing away all loose concrete and dirt. Apply a top coat of concrete patch or Quikcrete and broom it when setting so the stones are not slippery
Depends how deep, but you can apply a skim coat as its called of concrete and it will look like new, and it will last. As mentioned above broom it.
For your and everyone else's safety, it's really best to remove the and get new ones. Watch for sales, this is the time of year to get a good buy on then.