Stones can become unstable, or may crack due to poor laying technique, frost or a material problem. In whichever case, the stone needs to be lifted and relaid. Laying stones on a bed of sand is a more straightforward procedure than shown here, where they have been laid on mortar.
Use a pry bar to lift the old stone (Image 1). If it is laid in concrete, break it with a sledgehammer and chisel. (Block of wood provides leverage.) Wear protective goggles and gloves.
Chip away any old mortar and remove debris from the stone bed (Image 2).
Lay a new bed of mortar, leveling it off with a trowel (Image 3).
Position the new slab, checking that it is level with those around it. Tap it into place with a rubber mallet (Image 4).
Repoint the stone with mortar, or a dry mix as required (Image 5).