You can set the Pi to boot off an USD drive of you're trying to switch between OSes, but the Win10 you're talking about is the IoT Core edition, that's without any graphical interface and thoroughly slimmed down to the bare necessities.
It's not Windows as most people understand it. A fresh install of Windows 10 on the Raspberry Pi doesn't boot to the familiar Windows desktop. Instead, Windows 10 IoT Core will show users a single full-screen Universal Windows app. The system will only display the interface of a single app at a time, although additional software can be run in the background. Apps are loaded onto the Pi from a Windows 10 desktop machine.
