If you have downloaded Windows 10 and created a bootable USB drive, the next step is to boot from the USB drive to clean install Windows 10. There are several reasons why. These days, Windows ISO images of certain editions are as big as 5.3 GB so it is best to not use a DVD drive to install it.
If Windows 10 is already installed
First connect your USB drive on the computer where you want to do boot from and clean install Windows 10.
Now press the Win+X keys together to show a special menu. In the Shut down or sign out menu, keep holding down the Shift key and press Restart, and then let go of the Shift key.
Tip: You can also right click on the Start button to show this same Win+X menu but if you are using a Start menu such as Open Shell, it may show its own context menu and you may have to Shift+right click on the Start button itself too.
Windows will show a page with advanced startup and recovery options.
Choose there Use a device, then UEFI:Removable Device.
Your PC will boot from the USB drive so you can clean install Windows 10.
If your PC came with Windows 7, DOS or no OS
This is a little bit trickier but you can certainly figure it out easily. When you turn on your computer and it starts, you are able to press certain keys to enter the BIOS before Windows begins to load. These keys usually are F2, F9, F10 or Del key. Observe on the screen for any message that instructs you about which key you need to press to enter the UEFI BIOS.
Alternatively, some computers directly let you press a key without entering the UEFI BIOS to quickly do a one-time boot from another device such as USB instead of the internal disk. Or it may present such an option after entering the BIOS. Such keys usually are also from one of the 12 function keys – F1 to F12.
By pressing these keys repeatedly before Windows boots or after you enter the BIOS, you will notice some option related to Boot order or UEFI Boot devices. If you successfully enter the BIOS, look in its Boot section for options to change the boot device order or access a boot menu. This will allow you to boot from USB to clean install Windows 10.