After upgrading (I use this term in only the loosest sense of the word) to Sequoia, I'm very disappointed... I'd like to go back to Sonoma

My iMac... Retina 5K, 27-inch, 2020. I recently installed Sequoia and, after having done so, am so disappointed with some of the changes they made that I'd like to go back to Sonoma. I downloaded Sonoma from the app store and, upon completion, was unable to install (apparently because I still had Sequoia running on my Mac). Any easy way around this without having to save years of files to an external device or location?