Lenovo: How to boot from USB disk under Windows 8, Windows 10 – Idea/Lenovo laptops

Only Windows Boot Manager is available in your PC boot menu while trying to reinstall Windows 7 over your preloaded Windows 10 or Windows 8/8.1 via a bootable USB drive (or USB stick), you can apply the following steps to get system to boot from USB disk (or USB stick).

https://support.lenovo.com/es/es/solutions/ht500207