Create a fresh Windows 10 Pro ISO file with Microsofts Media Creation Tool. (I created a Windows 10 Pro 64 bit German ISO image)
Save the created image on your SmartOS global zone.
cp ... /zones/Windows10_64de.iso
Laptop (ThinkPad) Lenovo X1 Carbon 6th Gen (Type 20KH, 20KG)
If you are using your ThinkPad X1G6 with Linux in "Legacy only" mode, there's only one way how to update BIOS - you should use "BIOS Update (Bootable CD)". So this is a step by step guid how to do it.
fwupdmgr get-devices