Actually, you can do it all manually if you wish but, I won't cover that option here, as this means you know what you are doing and won't be needing to read this
I always used the cvs + build.sh method to upgrade or apply a patch on my NetBSD installations. I don't even remember if ten years ago (when I installed NetBSD for the first time) there was something like sysupgrade. 🤣
I think I will always continue to use that method to upgrade, as one of the reason I love NetBSD is the build.sh+mk.conf combo. It was just love at first sight.
That said, a very good guide. NetBSD needs something like this, as I always saw the handbook as something with too much informations for a newbie, despite still being a great resource. Sometimes something more "quick and dirty" can be more useful. 😃