When you get a new device and start up Windows 10 for the first time, the antivirus protection that comes installed with the device is your default security app. However, Windows Security is pre-installed and ready for you to use at any time.
If you want to use Windows Security, uninstall all of your other antivirus programs and Windows Security will automatically turn on. You may be asked to restart your device.
Note: In previous versions of Windows 10, Windows Security is called Windows Defender Security Center.