Very easy.
1. Boot pc, stop it and go directly into bios, usually DEL or F1, screen will tell you, soon as you see the screen, bash that key. If it tries to load windows, you went too far, start over.
In bios, look for boot options. Make sure USB or optical drive (whichever has the win10 on it) comes first in the order, before HDD/SSD. Save (F10).
Plug in the media (disk or USB)
Reboot.
Install will start from your new copy of windows, do a full, clean, new install, if it asks for format, are you sure, say yes.
Go get a drink and a sandwich, this will take a while.
Pretty much done, other than naming the pc etc, directions are on screen.