How to open old under cabinet halogen light?
What have you tried so far? On those type of lights, I've seen two different ways to get the light out. First method is to twist the glass counter-clockwise by pressing on it and twisting. It will then pull straight out giving you access to the bulb. The other method I've seen is where there is a tab and you put a screwdriver in and the center cartridge pulls out. 

Once you get at the bulb, remember to never touch a Halogen or Xenon bulb with your bare finger. The oils from your skin can weaken the glass of the bulb and shorten its life. Use gloves or a cloth to hold the bulb when removing it from the package and inserting into the fixture. 

This video looks similar to your photo and shows how to open it using the screwdriver method.