Cat III:
I did the 8.1 on a new ASUS laptop, which arrived with 8. First I had to do all the updates, so that it would in fact go out and find 8.1 on the update site. There were close to 100 updates to 8 alone.
After that, if I recall, the 8.1 showed up as an available update at Microworst's update site. The download was huge, something like 160 megs. (I don't know my "iso's" from a hole in the ground!) The whole process of updating and installing took all of 6 hours, on my DSL broadband line. I'm glad I didn't try to do it on my alternate 4g line, which has a monthly limit.
I have to admit, I was really surprised that when rebooted for the final time, 8.1 booted up and ran without a hitch.