How to get old versions of macOS - Apple Support Open the disk image, then open the .pkg installer inside the disk image. It installs an app named Install [ Version Name ]. Open that app from your Applications folder to begin installing the operating system. macOS Sierra can upgrade El Capitan, Yosemite, Mavericks, Mountain Lion, or Lion. OS X El Capitan can upgrade Yosemite, Mavericks ...