Start up your Mac in Windows or macOS with Boot Camp
After you use Boot Camp Assistant to install Windows on your Intel-based Mac, you can set the default operating system to either macOS or Windows. The default operating system is the one you want to use when you turn on or restart your Mac.
Set the default operating system
In Windows on your Mac, click
in the right side of the taskbar, click the Boot Camp icon 
, then choose Boot Camp Control Panel.If a User Account Control dialog appears, click Yes.
Select the startup disk that has the default operating system you want to use.
If you want to start up using the default operating system now, click Restart. Otherwise, click OK.
You can’t change your startup volume to an external drive while you’re using Windows.
To learn how to change the default operating system using macOS, see Get started with Boot Camp.
Restart in macOS
In Windows on your Mac, do one of the following:
Restart using Startup Manager: Restart your Mac, then immediately press and hold the Option key. When the Startup Manager window appears, release the Option key, select your macOS startup disk, then press Return.
If you’re using a Mac notebook computer with an external keyboard, make sure you press and hold the Option key on the built-in keyboard.
Restart using Boot Camp Control Panel: Click
in the right side of the taskbar, click the Boot Camp icon , then choose Restart in macOS.This also sets the default operating system to macOS.
For more information about restarting in macOS, see the Apple Support article Restart your Mac in macOS or Windows.