Assuming the door is closed, if you have lost the keys you will have to call a locksmith. The cylinder (barrel) is secured by a screw on the closing edge of the door so you can't access it unless the door is open.
I just replaced the cylinder on my door and it's easy to do but the door has to be open. Even when open, you probably need the key in the lock in order to move the cam slightly so that the cylinder can slide out.
EmCee, January 2010