Careful not to pierce the balloon at this stage.
The edge of a knitting needle is pointed enough to get into that tight little spot. But it is not sharp enough to pop the balloon. You can use it to loosen the knot and untie it.
Focus on particular loops of the knot when pushing and pulling. The goal is to loosen and grip a single strand of the knot.
This is easier if your fingernails are long.
This knot is called a lark’s head knot. The goal is to use the rubber band as a tightening agent. For increased support, you could tie the rubber band as well. This could be just as challenging to untie as a regular balloon’s knot.
Use scotch tape for the best results.