You dont have to have windows 10 to update the bios, just a usb stick.
Heres straight from the Dell website:
Updating the BIOS from BIOS Boot Menu (independent of operating system)
Note 1: Before updating the BIOS, ensure that you suspend BitLocker encryption on a BitLocker-enabled system. If it is not enabled on your system, you can ignore this step. For information about how to disable BitLocker, see How to Enable or Disable BitLocker with TPM in Windows at support.dell.com.
Note 2: Do not turn off the power or interrupt the BIOS update process during the update.
Installation
1. Copy the downloaded file to a USB drive. The USB drive does not need to be bootable device.
2. Insert the USB drive into any USB port.
3. Power on the system.
4. At the DELL logo screen, press F12 to access the one-time boot menu.
5. Select BIOS Update in the section.
6. Click Flash from file to browse the USB drive to locate the downloaded file.
7. Select the file and click Submit.
8. Verify the existing system BIOS information and the BIOS update information.
9. Click Update BIOS.
10. Review the Warning message and click Confirm Update BIOS! to proceed with the update.
The system restarts and displays a progress bar at the Dell logo screen.
The system restarts again when the update is complete.
When i updated my 5070s i used an 8 gig usb stick formatted as fat32