Click Restart…. It's near the bottom of the drop-down menu.

Click Restart when prompted. Your Mac will begin to restart.

Place your Mac in recovery mode. Immediately after clicking Restart, press and hold the ⌘ Command key and the R key simultaneously until greeted by the "Utilities" window.

Select Disk Utility. It's the grey hard drive-shaped icon in the main window.

Click Continue. This is in the bottom-right corner of the window.

Select your Mac's hard drive. On the left side of the window, click the hard drive on which your Mac's operating system is installed.

Click Erase. It's a tab at the top of the window. A pop-up window will appear.

Click the "Format" drop-down box. You'll find this on the right side of the page. Doing so prompts a drop-down menu.

Click Mac OS Extended. It's in the drop-down menu.

Click Erase. This is in the bottom-right corner of the window.

Wait for your Mac's hard drive to finish erasing. The erasing process will take some time, so just make sure that your Mac is plugged in and charging to prevent it from accidentally dying.

Click Done when prompted. Doing so completes the erasing process.

Click Disk Utility. This menu item is in the top-left side of the screen.

Click Quit Disk Utility. It's at the bottom of the Disk Utility drop-down menu. Doing so returns you to the main Recovery window.

Select Reinstall macOS, then click Continue. MacOS Sierra will begin reinstalling itself on your hard drive.

Follow the on-screen prompts. Once MacOS Sierra has been reinstalled, you can proceed with setting up your Mac (e.g., choosing a language and connecting to Wi-Fi).[2]