Try the "Boot Options" key - Normally its F11 or F12. In this list of available devices to boot from, the last one or second to last usually is an option to enter the BIOS from there.
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=== Update (03/04/2018) ===
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You don't need to do this to get Windows 8.1 installed.
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Windows 8.1 is a service pack consisting of 3 files you can install directly from Windows 8.0.
Try the "Boot Options" key - Normally its F11 or F12. In this list of available devices to boot from, the last one or second to last usually is an option to enter the BIOS from there.