Apple provides the full installer for older versions primarily as DMG disk images
: Before proceeding with any OS installation or upgrade, it's highly recommended to back up your Mac. Use Time Machine or any other backup solution you prefer. download macos 1012 sierra full
: If your Mac is compatible, you can find the link to the Sierra App Store page which will open the "Software Update" or "App Store" app to begin the download. Apple provides the full installer for older versions