Loose knot

My wife gave me a black badger Col. Conk brush several years ago. This spring I bought a boar bristle brush and have been using it for a few months and decided to go back to the badger for comparison. While I was swirling it in the soap, I felt a clicking in the handle. When I squeezed the water out of the brush after the final rinse I felt the knot move. It moves back and forth in the handle.

Is there a way to fix this without removing the knot from the handle, and if not, how do I remove and reset the knot?