[NAME]  Are you trying to run the BIOS update using SupportAssist in Windows, or just manually in Windows? Try doing the update by putting the BIOS .exe file on a (non-bootable) USB stick. Plug that stick into XPS 8950 with power fully off. Power on and tap F12 to open the boot menu. Select the BIOS Flash Update option and follow the prompts. Additional details about this method are under "Updating the BIOS from BIOS Boot Menu (independent of operating system)", here.  
The latest updated XPS 8950 BIOS was posted today (October 10th), Version 1.7.0. I downloaded it. When starting the install, I get the UAC prompt for permission, I respond Yes, and then system freezes. BIOS does not install. In order to gain access to system, I need to power off and then back on.
XPS 8950, freezes trying to install latest BIOS 1.7.0