I recently upgraded from 3.0.12 (I think) to 3.3.3.
I am a tinkerer and no pro when it comes to phpBB.
I found the instructions absolutely spot on:
https://www.phpbb.com/support/docs/en/3 ... upgrade31/
I was done and dusted within 3 hours, and that included me downloading FileZilla and learning how to copy the files across to my site, as I had never needed to do this before - my site was an "out of the box" install done when I purchased the domain and hosting package 3 years ago. I had never otherwise needed to touch anything about the site.
Very impressed with how simple it was to update.