There are two schools of thought, depending on how mission critical your website is.
1) backup the database and all the files on the server
2) install the update
4) Fix anything that's broken, but if that fails, delete the files and database tables, and restore your backups.
5) Wait a few weeks for developers to address the issue, then try again.
Have your own staging server/subdomain where you can set up a private copy of your wordpress site and do all the testing you want without affecting the live site. I tend to do this on a local workstation, rather than a live server (just in case there are issues).