First, you'll need the Morph-O-Ray, which can be found by exploring planet Oltanis (note: it can't be obtained from Gadetron Vendors).
Once you find it, travel to Rilgar and find one of the several tanks throughout the planet, you'll need to get very close before you aim and fire the Morph-O-Ray at the tank. As the tank is quite large, it will take a few seconds for the tank to turn into a chicken, so remember to hold down on the trigger.
As it does take several seconds and the tank will be firing on you, it will be easiest done after you've got the ultra nanotech from planet Orxon.