How can I install Arch Linux on a computer that already has Windows 10
Have you tried turning off fast reboot in Windows 10. I had the same problem. Fast reboot is turned on by default in Windows 10, which skips ability to enter BIOS.
How to Disable Fast Startup
After turning it off you should be able to enter BIOS.
You can restart with advanced restart option in windows 10 and if the bootable usb is detected, you can choose to boot from the live image. shortcut to boot to bios can be anything from esc/f10/f12 etc.