There just glued on so you need to remove, clean and re glue. Be careful if using Super glue, if any drips out if will ruin whatever it drips onto.
Many years ago I had a lose left grip on a Yam rd400, kept saying I must fix that, pulled in for petrol, pulled the bike onto the mainstand, grip came off in my hand, bike ended up in a heap by the pump, one nicely dented tank!