The above will install the original OS X that came with your computer from the factory is you use the proper shortcut keys as provided in Step 1. To install the original factory version when the computer was new use Command-Option-Shift-R.) (To install the version of OS X that was currently installed use Command-Option-R. Immediately after the chime hold down the Command-Option-SHIFT-R keys until a globe appears. You would either need a copy of the Sierra full installer application or re-download the installer from the Purchases page in App Store or have made a bootable flash drive installer for Sierra. Step 2: Shut down the Mac and wait for 1-2 minutes. Step 1: Insert the macOS bootable USB drive on a USB port of Mac computer.
Then you need have a way to install Sierra. How to Install macOS on Mac from USB Drive (Normal Way) If the target Mac computer already had a macOS or OS X on it, then installing macOS from USB is straightforward and this is called the normal way. I used a thing called DiskMakerX7 to create a bootable 16GB USB drive named 'Mac OS High Sierra Install Disk' It seemed to boot OK, so I selected 'Install MacOS' Now, when I boot the iMac (usb not inserted), all I get is a 4-choice menu: Restore a Time Machine Backup, Install MacOS, Get Help, and Disk Utility. You cannot downgrade without first erasing the drive. Use Command-Option-Shift-R to install the original factory version when the computer was new. macOS Sierra is Apple's latest desktop operating system, which succeeds OS X El Capitan and adopts a new name to bring it in line with iOS, watchOS. That will make your Mac run blazingly fast for a few months or more, and many people report that it fixes longstanding problems they’ve had too. Use Command-Option-Rto install the version of OS X that was currently installed. To download High Sierra for use with a bootable installer, be sure to download from a Mac that is currently using High Sierra, Sierra 10.12.5 or later, or El Capitan 10.11.6. The idea behind a macOS Clean Install When you install a new major version of OS X, or after some time has passed, it is a good idea to do a clean install (that is, to erase the disk you are installing OS X onto as part of the installation process). Select Reinstall OS X and click on the Continue button.Quit Disk Utility and return to the Utility Menu.(Install the latest macOS that was installed on your Mac). Click on the Apply button, then click on the Done button when it activates. Boot up, and immediately hold down the key combination Command + R.