My assistance is to say ...

Don't fix what's not broken, unless you like bricks. :Smile:
Or said another way ... Don't fix it until it's broken. 

See this page and its URL-Links:

https://www.dell.com/community/Inspiron-Desktops/Dell-Inspiron-3847-shut-down-during-BIOS-update-and-will-not/td-p/6068453

If you have an actual reason to update the BIOS (and you think it's worth the risk) feel free to continue. Someone here might even be able to help you with that:

 http://www.dell.com/support/home/us/en/19/product-support/product/optiplex-990/manuals

and all files are here:

http://downloads.dell.com/published/pages/optiplex-990.html

Sometimes BIOS updates on these older machines just doesn't go so well. Also, with all the reported problems users have, I sometimes wonder if all the files sitting on the Dell servers are still perfect and usable for their marked purpose. Be careful.