Welcome to the community, @Stefan!
It would be really helpful if you share your system's specs, especially the motherboard and the storage configuration. I'd recommend you go to the motherboard manufacturer's official website and download your mobo's model manual from there, in case you cannot find it at home. There you should be able to see all the BIOS options and settings, and how to access them. Another thing that could help you is
resetting BIOS. It will get your settings back to factory defaults, so you should be able to access and change the boot order again. Make sure you re-configure all the manual changes you have made so far in the BIOS after the reset.
Hope this helps. Keep me posted if you have more questions!
SuperSoph_WD