If you want to regrout the bricks then you will first need to remove the existing mortar. This can possibly be done using a grout saw, you need to remove at least 1/4" deep in order for the new grout to have the side surface to adhere to. Here's a link that'll show you how to remove grout, though if it's mortar it may take a bit more muscle to get it out.
http://www.rd.com/familyhandyman/articles/199711/main/
If you want to tile the brick instead, then here's a link that'll show you all the details of how to:
http://ronhazelton.com/html/407a.html