Get a thermostat. I recommend the Herpstat by Spyder Robotics (http://www.spyderrobotics.com/home/products.html). I like the Herpstat 2 because it has two channels and can control my UTH and tank heating (CHE or RHP) separately.

You want the hotspot to be around 90 at the hottest point, which means it needs to be regulated. Having 140 at the bottom of your tank means if your snake tries to burrow down there, he will get burned! This needs to be fixed right away. The cheapest method is to buy a cheap thermostat on Amazon (as someone already linked) or get a lamp dimmer from your local hardware store and plug the UTH into the dimmer. Bring the dimmer down to the appropriate temp using your heat gun directly on the glass where the UTH is mounted.