I fitted one just last week - just undo the 4 screws on the black plate of the lock unit - then, using a mallet, tap the brass barrel from the other side and it should come out (the hole size on the fitting instructions is very tight - I had to bash the lock through to fit it) The black handle and brass disc are both just screwed to the face of the door, slipping over the brass barrel - again a tight fit. The retaining "top hat" obviously just unscrews from the foor frame.
The comment about the drill bit size is apposite - I used a 25mm Forstner and "worried it bigger" with the drill and then used a Surform round file to ease the size - but still tight.