To stop the engine, you would simply turn the key in the ignition to the "off" position.
If someone *has* installed an aftermarket kill switch, its location and operation depend entirely on where and how the installer wired it. It could be anywhere – under the dash, under the hood, even hidden somewhere completely inconspicuous. There's no standard location. You'd need to:
* Look for obvious signs: Check under the dash, near the steering column, and under the hood for any switches that look out of place or don't have obvious factory labels.
* Check wiring: If you're comfortable working with car wiring (and know what you're doing – safety first!), you could try tracing wires from the ignition system to see if you can find an unusual interruption point. This is dangerous if you don't know what you're doing and could cause damage.
* Ask whoever installed it: If you didn't install the kill switch yourself, the only reliable way to find it is to ask the person who did.
Without knowing if a kill switch was added and where it's located, there's no other way to answer your question.
