When startup is complete, you’ll see the Utilities window. Hold down the Command+Option+R keys until you see the Apple logo or a spinning globe. Follow these steps to reinstall macOS from the internet: Turn on or restart your Mac. Select the Restore partition next to your startup disk. Shut down your Mac, then hold Option while it turns on.
Older Macs, running Mac OS X Snow Leopard or earlier, might need to boot into the Restore partition instead of macOS Recovery. This is to be avoided, turn off power to abort, as the macOS it will install is the earlier version that originally shipped with your Mac.
Caution: If you use (Command + R) and a spinning globe of the world appears, the Mac has not used the Recovery partition but is using Internet Recovery.Either way, if you’ve got a slow Mac, you’ll be seeing that beach ball quite a bit - and you don’t even get a suntan to go with it. Wikipedia calls it the Spinning Pinwheel, and the official name for it is the Spinning Wait Cursor. People call it the Spinning Beach Ball of Death, or the Rainbow Wheel.
