When working on projects, I often want to strictly control the version of WordPress that is running. However sometimes one of the other administrators on the site sees the “WordPress 5.9 is available! Please update now.” message and clicks it, thereby prematurely updating to a newer version of WordPress core. This notice can be disabled.
These are the two ways I have used to rollback WordPress to an earlier version. Using either technique, you should make a backup copy of your files and your database first!
I setup a site for WordPress Core development with Local by Flywheel and documented the steps. If you’re trying to do the same, this might be helpful.