You can try and let it keep running but eventually you may just have to reboot the system. Read this Dell support page as I believe you have the Skylake chipset so BIOS Recovery should work on your system if you find that the system will not boot due to a failed BIOS update.
I have Inspiron 3650. Support Assist recommended upgrading BIOS to 3.12.1 BIOS Update Running Over 2 Hours.  The last time I updated it, I thought it took 15 minutes. What to do now?  Let it keep going?  Reboot & start over? What is the likelihood it won't reboot if I restart PC?
BIOS Update Running Over 2 Hours