It depends on how wide the joint is. If it's a half inch or less, clean it out well. Remove all of the old caulk. Install a non-absorbant backer rod and fill it with a gun grade urethane caulk. Keep the caulk bead within the dimensions recommended by the manufacturer for depth and width. If it's wider, clean and remove all of the old caulk. Install the backer rod and fill it with a pourable urethane sealant.