You just need to be able to support the bike by something other than the swingarm/wheel so the sring is not under pressure and then remove the rear shock. You can use an abba stand for this, or even axle stands under the pegs might be possible
Once you slacken off the preload you should have enough slack that you can slide out the ring at the bottom of the spring, there's a gap in the ring jus big enough for the shocks damping rod to pass through, and then the spring will slide straight off the shock.
Slide your new one on, stick the ring back in, tighten up the preload to keep the ring in plce then refit to bike and set the preload properly for the appropriate sag.
You might be able to leave the top of the shock attached to the bike while you do all this at the bottom of the shock, but I've not fitted my new spring yet so I don't know for sure,