I've repaired and replaced quite a few riv nuts in carbon frames. The process is pretty much the same for carbon or aluminum frames. The main thing is not to enlarge the hole in the frame that the riv nut sets into any larger in diameter than what it was originally. I actually replaced 2 riv nuts in a carbon Cannondale Super Six, which would be the same process as your Six 13. One riv nut had a bolt broken off inside it and the other had the threaded portion break off inside the frame, so both needed to be replaced. Sometimes if the riv nut is just loose, you can reset it as the others have suggested. I made a video of the repair process in you're interested in seeing the replacement repair demonstrated...