How can I boot back into macOS if Windows install in Bootcamp fails?
Shut off the computer. If you can not shutdown the computer, then hold down the power button until the computer shuts off.
Start the computer and immediately hold down the option key until the Startup Manager appears.
Select the icon represent the macOS operating system volume.
Hold down the control key and then select the circular arrow below the icon.

Note: On really old Mac models, you will only get a straight arrow when holding down the control key. If this happens, you will have to set the default startup operating system from the "Startup Disk" window in the System Preferences application.