4. Charge your device
If your device experiences a power failure during installation, you most likely won’t be able to boot macOS.
If you’re using a laptop, make sure it’s plugged in and charging from the time that you begin downloading the High Sierra installer. Give your device time to charge the battery fully. If your device does power off for any reason once installation has begun, try to restart the process via the Mac App Store. Failing this, use macOS Recovery to reinstall Sierra (see bit.ly/macos_recovery).