uPVC Handle and Door Lock.

It's about the simplest and easiest job you can do. I remember when I first tried to replace mine, I took out my screwdrivers and started to take the complete side off the door. Then I said 'stop, lets look up Youtube'. It is literally a 60 second job. First take out the screw shown in red below - on the side of the door (just make sure you take out the right screw, look at its position relative to the center line of the barrel). Then turn the key until the blue part is recessed under the barrel, you can't withdraw the barrel if this part is sticking out. Voila, then just pull out the complete assembly (from the inside of the door). They come is several different lengths for different door thickness. Just bring the one you took out to your local hardware store. They can be got with a key in the inside or a 'thumbscrew' , i.e. you don't need the key to lock from the inside.

Agree with the above comments but definitely use a screw driver and not a drill and bit to remove and refit the screw. Also get a screwdriver that is the perfect fit to the screw as they are soft enough heads on them.