Changing the battery is easy - although fiddly, and not good for your back! It's a heavy lump, and lifting it out of the car is a real recipe for problems if you have any form of back issues. My advice is bite the bullet and pay the little bit extra to have the likes of Halfords do it for you, which also saves you the hassle of disposing of the old one.
@ Giosabcsl - out of interest, what is the "proper" and "faster" way please?
Sure thing. So the above link you saw from Audizine that the more proper way.
The faster way doesnt involve any mess and literally can be done in under 5 mins. All you need is a syphon, turkey blaster or some form of suction item.
Then all you do is suck out as much old fluid out of the power steering container. Then u fill it up almost to the top of the container start the vehicle and turn the steering lock to lock a few times or you can go out and drive the car which ever you prefer. Then return, remove the lid suck the fluid out and repeat the above. Until you see nice green fluid. Literally that easy.