Is there already confirmation that the latest bios supports the Ryzen 3000 series? So even if you had a way to upgrade, the CPU might still not be supported. For the B450-f in particular, there are no clear release notes for the latest bios, so we are left to guess.
Edit: see here, it should be ok:
https://www.asus.com/News/EtaH71Hbjuio1arVAsus has a bios flashback functionality that allows you to update the bios without CPU/mem but you need to follow specific instructions
https://www.asus.com/support/FAQ/1030111