Apple doesn't want to give you a choice but you can still do it: 1. Use some mac to download an older OS installer. The links are hidden but you can find them

Apple doesn't want to give you a choice but you can still do it: 1. Use some mac to download an older OS installer. The links are hidden but you can find them in some forums etc. 2. Create a bootstick - either manually or with software like dosdude's really nice patchers (find them here: http://dosdude1.com/ http://dosdude1.com/) 3. Backup your stuff (not only time machine as it would reinstall the new version of macos again) and install the old macOS to get your apps and files back on it This is something I really hate about Apple - why can't they just provide some official download links that keep working? With Apple I have a love-hate-relationship because some products (MBP until 2016/17, iPhone 5s/SE) and software (keynote, pages, lot of parts of the OS) are awesome while their software policy and planned obsolescence screws you hard if you just follow their lead as a simple user. You always have to take care not to update too early, use workarounds to integrate devices with different Apple software (like with airdrop) and if you brick devices there is few ways getting back to business (no helpful error messages like the icons they show on boot when something goes wrong or messages from their system diagnostic like "HDD is damaged" while it's just the cable).