Having done this yesterday, it is fresh on my mind. It is not difficult, but there are a couple of factors that make it annoying.
To swap out a supply, you need to remove the top right encoder panel. To do this you need to remove not only the visible allen head screws but also a set of screws on the right hand side. To get to those screws you must remove the end cap (or in my case, the fader pack that was bolted to the right hand side). Then unplug it and set it aside.
Main units have three power supplies but fader packs have only two. Two of the supply types can be removed with the access provided by removing the encoder plate. There is not much space to access the screws holding the power supplies, so ideally you want a stubby #1 Philips screwdriver for that. The first time I did the job I sawed an extra
Lectrosonics screwdriver in half to get a short #0 Philips.
The LED supply has an additional heat sink that is screwed to the power supply from the top as well as to the chassis from the bottom. So for that supply you must remove three screws from the bottom as well. The D-Control service manual linked earlier in the thread recommends having the unit on two tables so you can access these screws from underneath. I didn't have two tables handy so I tipped the unit up to get to these screws. Doing this by myself was precarious. If you can have a second person help it would be better.