A search will bring up more on that job than you'll have time to read. The manifold has to come off and if all of the plugs come out in one piece, it's a pretty easy job. However, there are lots of stories about glow plugs breaking off in the head. Many have had to remove the head to fix it. I may have been lucky, but I was able to drill mine out.

Why are you wanting to change them? Have you tested them with an ohm meter and found some weak or bad?

If your fuel lines are original on the car, then it would probably be wise to change the fuel lines, O rings and washers while the manifold is off.

Len