How to disinflate a helium balloon to reuse it?

Is the balloon a latex or Mylar one. If it is the Mylar (plastic like and shiny) then you can insert a strong plastic straw into the opening until it is about an 1 1/2 inches past the wide area, and this will let the helium out. If it is the latex kind then you would need to very carefully undo the knot. You would need to be sure that you do not make any kind of holes in it. I have never tried to undo the knot on a latex balloon, but I have been able to deflate a Mylar one. Good luck.