Not sure there's a safe way to 'let the steam out' of even a small volcano, much less a super volcano like Yellowstone. That thing is huge, and if it undergoes a major eruption it would cover everything east of it in several feet of ash, all the way to the east coast. I would imagine that doing something to relieve the pressure would be like trying to let the air out of an over-inflated balloon with a sharp needle.

KABOOM!

As I understand it, the magma in the chamber below the park contains immense quantities of dissolved gases, which are held there only by the tremendous pressure of the magma. Releasing that pressure would allow the gases to escape, similar to opening a soda bottle after shaking it. Doesn't sound like a good idea to me.