From looking at Dell's website the only BIOS update options seemed to be for supported operating systems (Windows, Red Hat, etc).
How can the bios be updated using a bootable USB or via the IPMI?
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Dell provides .iso files with all BIOS and other firmware updates for their server products, called "Server Update Utility DVD ISO". You can burn it on a DVD, load it on the server and install them directly from the management interface.
You can find the .iso in the "Systems Management" section of the support page of the product.
There are some other .iso files available that could be helpful as well.
For the PowerEdge 1950 the current URL is: http://www.dell.com/support/home/us/en/28/product-support/product/poweredge-1950/drivers