make sure you backup onto external medium any data you want kept first.
but if you plug in your USB stick with Windows on it, and have told the BIOS to include USB as a boot option (or press F12 or F9, can't remember the exact key to bring up the boot choices during startup) then that start the installation straight from the stick.
make sure you have your Windows key handy and the USB stick needs to be set have a bootable ISO image on it, not just simply coping a Windows version to it.