How do I change my BIOS back to UEFI?

[Microsoft Windows]

My OS disappeared overnight. Was working fine the day before.I didn't mind as I needed an upgrade anyway. I changed the BIOS from UEFI to Legacy in an attempt to install a new Windows and I can't change the BIOS back to UEFI. The option is not there. I've reset to defaults with no luck. I have an ASUS P8P67.

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