To reset the MacBook Air to factory settings at startup, use Erase Assistant to wipe out the device. Restart the system and then go to Apple Menu. After that, click on Shut Down. Hold the power button till you see Loading startup options on the screen. Click on Options and select Continue. After that, you need to follow the instructions available. Go to the Recovery app window, and choose Reinstall for your macOS release. Select Continue and do as instructed.