I actually haven't removed mine as it was pretty close for me out of the box. That said, I'm sure it's no different than any other shock. Just back off the preload collars all the way and remove the clip from under the bottom collar.
A spring compressor is nice if you don't want to disturb your preload settings, but not a necessity as far as I'm aware.
HERE is a link to the Ohlins manual. It doesn't mention the spring removal but has some usefull information regardless.