The question is likely obviously answered as everything. But I was just curious; Is there a guide available as far as what should be backed up both files and database related prior to an upgrade attempt?
The only things you need to back up are the database, the /data and /internal_data directories, which is where avatars and attachments are stored.
The XenForo files on the server you can download at any time from here.
If you have any add-ons installed, then as long as you already have copies of those somewhere, they are effectively already backed up.