If you have the key, remove the ignition cylinder and, depending on the lock style, either remove the tumblers ar file all of the locking slides flat with no key in it - then simply reinsert the lock cylinder.
If you have the key, remove the ignition cylinder and, depending on the lock style, either remove the tumblers ar file all of the locking slides flat with no key in it - then simply reinsert the lock cylinder.