Thanks again to @arvidjaar …
The BIOS update went fine - copied the .exe file to /boot/efi (while in TW) and rebooted to the Dell boot options (F12?) and selected BIOS Update, and when I clicked Select File, it opened right up at /boot/efi 
(I didnt know the BIOS executes the .exe, why I thought I need DOS or Win)
The “retbleed” message still shows after BIOS update, so I set
“VBoxManage modifyvm “VM name” --spec-ctrl on” and all is good.
DONE !! ( RESOLVED )