How can you regrout a stone fireplace?

If the Stone Are still adhered to the fireplace like they should be Just get the color grout you want and grout it like you would grout tile. If the stone needs more support or some have fallen off you need to use brick mortar and place the ones that have come have back on and wait 24 hours then grout them with the tinted brick mortar.

What would be the reason for doing this?

Is the mortar chipping off, falling out, or do you just not like the color?

Please post a picture so that we can see what the situation is.

https://www.youtube.com/watch?v=En739D8Dqcs

Try to dig some of the grout out then add new

remove old grout and add new grout