I was thinking you meant a collar. Support the other end of the rivet on something tough enough then smack it with a centre punch. Turn over and repeat.
Edit: just to clarify, the handle and the blade are solid and not loose. It’s the metal ferrule/end cap that is loose. It’s not going to come off, as it’s captured between the blade and the wooden handle, but it moves.
It’s riveted, rather than having a tang bent over at the end. I guess I could squeeze it, or drizzle some glue in, or both.