Skip Support Assist.  Download latest BIOS to a blank flash drive and then boot from it.  Or download directly to PC.  Please see articles and be sure to follow any instructions to the letter. By the way, Dell Support Assist should work as it worked for me in the past and I ran into an instruction article on it for updating BIOS.  However, DSA has been hit or miss for other users. Find a driver for your Optiplex 5000  [URL]  [URL]  For internet connection, Ethernet is more reliable than USB dongle.
Hi A newer Dell Optiplex 7000. We have several of these pcs and only one pc so far has this issue. When trying to reset pc via SupportAssist  in BIOS, download begins but after a few minutes of downloading it suddenly comes up with 'Can not connect with backend...please check your internet connection'. We updated Support Assist to latest 5.5.xxx but still no luck. How can we fix this issue with SupportAssist? Thanks Regards  
SupportAssist: Can not connect to backend