Deisel Clio 3 battey replacement question

[cannondale]

The battery on my dad's 2006 clio deisel appears to be flat as there is only some lights coming on on the instrument panel and it wont attempt to turn over. We have tried to jump start it but it's not having it. The battery has been dead for about three days so might not have had much chance to jump it.

The question is how to do you change the battery? The positive terminal looks almost impossible to remove!!

Also the magic eye bit on the battery looks a bit yellow. Think this means the battery has had better days. It is only a 2006 car so this in my opinion seems to have been a short time for a battery to expire.

Thanks for any advice given.