To be honest it's very easy to change a lock yourself.
All you need is a screwdriver. Unscrew the handle from the inside. Then you should see two screws which when loosened will relaease the round cover from the outside of the door.
Then, on the side there is another screw which you need to remove and voila, the lock comes out.
Take this down to a local ferreteria (DIY store) and they will give you one of the same size.
Of course, you need someone to mind your property whilst you go down to the shop!
I can't remember how much they cost though.
Justin