Can’t access to USB live system

Hello. As reported at The latest grub package update needs some manual intervention, I have this issue too. The problem is that I don’t have access to the boot manager by pressing the keys “fn” + “F12”. How can I boot by USB stick?
Edit: my laptop is a Lenovo ideapad

Hi fran,
can you open the BIOS with fn + F2? And then check there the Boot-Settings.

Hi, pressing fn + F2 doesn’t work, I don’t know why. Solved by changing the bios from uefi to legacy (now I see the USB drive)

How did you change from uefi to bios without accessing the bios?

I pressed a combination of keys many times (ctrl + fn + alt + f1–or f2, don’t remember), after a reboot I got access to the bios

I have 4 Lenovos in use and F12 alone is sufficient for all of them, i.e. without the FN key.