Assuming you have the keys for the current lock (as in you haven't locked yourself out) it's a really easy job. You most probably have a euro cylinder lock which will be held in with a single screw. It's literally a 5 minute job. Just make sure you get the correct length cylinder.
I've literally just done this very thing today. If you have a screwdriver and the keys then it's a five minute job + going to the shop to get a £6.50 replacement. They're designed for easy swapping.
What have people paid to have a single lock in a upvc door changed before?