So if you have windows 10 installed but you want to do a fresh install you can HOLD SHIFT , click start --> reboot still HOLDING SHIFT.|
You can release SHIFT when screen become Black.
then press TROUBLESHOOTING and you can choose what you want to do. Reset PC will let you chose if you want to reset holding personal file or completely remove all.
If you have installed a different OS and you want to restore it, the better way is to download windows 10 professional 64 ad make a bootable usb for uefi bios.