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.
The "probably" and "just make sure" parts of your response are exactly why I'm getting someone in who will get rid of those and provide a guarantee. Anyone else used a locksmith and can compare costs?
What have people paid to have a single lock in a upvc door changed before?