How can I format my NVME drive?

After an update from Dell, I can no longer boot into my BIOS (F2 and F12 do nothing).
Before disaster struck I had managed do go through Windows to the BIOS and switched from UEFI to Legacy (for some Linux experimentation).
And that's when it happened (I had also just installed Dell's Newest BIOS...)
I tried to re-install Win 10 through the media creation tool, but was unable do to the partition being GPT whatever that means. How can I format the NvMe drive to a suitable partition to reinstall Windows?
I formatted (and thus lost all data) from the install part of the creation tool, but I still get the same error

When you first boot into the Windows setup USB flash drive, press shift + F10 to open a command prompt. Then run:
diskpart
list disk
select disk # <-replace # with the actual number for the SSD
clean <- this will erase the disk selected above, make sure it is the SSD!
convert mbr
exit
exit

Then proceed with the install. When you get to the screen that shows drives and partitions, just highlight the unallocated SSD and click next.

How can I know the SSD number? Many thanks for your help!

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It's drive partition 3. But shift-F10 isn't working for me.

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Made it to the command prompt, there was a huge C:\ icon on the choices. Did as you said, now will try to install!