Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Just press and hold the power button on your Mac until it turns off.Be sure you backup your files to an external drive or second internal drive because the following procedure will remove everything from the hard drive. If you're selling, giving away, or trading-in your Mac and want to leave it in an out-of-box state, don't use the setup assistant or your previously connected Bluetooth devices, if any. After restart, a setup assistant guides you through the setup process, as if you're setting up your Mac for the first time.After joining a network, your Mac activates.To select a Wi-Fi network, use the Wi-Fi menu in the upper-right corner of the screen. You might be asked to select a Wi-Fi network or attach a network cable.When reconnecting to a Bluetooth keyboard, you're asked to choose a language. If the accessory doesn't connect within 30 seconds, turn the accessory off and back on. If it needs to reconnect to a Bluetooth accessory such as a keyboard or mouse, you might be asked to turn on the accessory. Your Mac restarts and shows a black screen or progress bar.Click Erase All Content & Settings to confirm that you want to proceed.If you're asked to sign out of Apple ID, enter your Apple ID password and click Continue.Click Continue to allow your settings, media, data, and the other listed items to be removed.

Or click Continue to proceed without making a new backup. If you're asked to back up your Mac before resetting it, you can open Time Machine and back up to an external storage device.Enter the password that you use to log in to your Mac, then click OK. An erase assistant asks you to sign in with your administrator credentials.After choosing Erase All Content and Settings, follow these steps to continue:

How to erase everything on macbook air