Make sure you act fast when you notice your grout is crumbling or failing. Failed grout allows water underneath the tiles which causes the damage to spread rapidly. If left unattended, your whole surface may fall apart.
Before you being, check to see if your tiles are still properly attached to the substrate. Several loose tile in an area suggests the mortar has failed or there are moisture problem with the substrate. If that's the case, regrouting will not help; your only option is to retile.
The first step in regrouting is to remove the old grout. Using a grout saw, scrape out old grout. Take care not to scratch your tiles or chip their edges. You may choose to only regrout the damaged grout lines; but for the best results, you should regrout the entire surface.
Next wash your surface by vacuuming the debris and washing it down using a solution of equal parts water and vinegar.Lastly regrout the surface the same was as you did during the initial installation.