Drain and drop the pan, the sensor is on the passenger side, you'll see the plug coming off of it. You turn it to the right I believe and pull down, when it comes out, about a quart of tranny fluid will come out as well. Would also be a good time to change the screen, filter, and fluid as well.
when you put the new one in, you will get a code for high voltage at the sensor for a few days, this goes away and doesn't harm anything. Also, check the wiring for nicks and scrapes, if you see any replace the harness.