There are two ways you can change the lock cylinder without a key:
1.(PREFERRED) Have a locksmith either pick the lock or make a key.(if he just picks the lock, you can turn the cylinder enough to depress the release, and get a new cylinder)
2. Get a large flat screwdriver, vise grips and a hammer. Stick the screwdriver into the key slot, drive the screwdriver in a little farther with the hammer. Use the vise grips to turn the screwdriver and force the cylinder to turn.(may take several attempts and could break screwdriver.) Once the cylinder turns, depress the release and remove the cylinder.
If you have any other questions, please don't hesitate to ask.