how to take out old lock?

Assuming you have not lost the key. Look at the inner edge of your door adjacent to the lock. You will see a screw head. Remove the screw which is quite long. Insert your key into the lock, and turn slightly so you can pull the lock. While pulling turn slowly to line up the lock tongue with the lock slot, and it will slide out.
Fit new lock in reverse order.
If you lost the key, the lock will have to be drilled out.

Steve
May 2010