The other poster is correct in that these can be adjusted in place if you have a spanner wrench of the correct size (or a suitably adjustable one) and in your case it's easier than in many scenarios because there's very little in the way.
This leads us to point two which is I prefer to do at least the rough preload adjustment in advance if I have the shock off the bike already anyway, for instance in the case of my Orion when I had to replace the rear shock, since that's buried within the center of the frame with all manner of things around it like the airbox and rear fender liner, etc. Swinging a wrench in there is nigh impossible. On that one I have to use the "wrong" way, i.e. get a screwdriver in the slots and tap it with a hammer, because using a spanner in that position is literally impossible.
When adjusting a shock's preload rings while it's dismounted, you will probably find it helpful to clamp it in a vise. Otherwise you will need three hands.