Install the racadm tools on your local system (http://en.community.dell.com/techcenter/systems-management/w/wiki/3205.racadm-command-line-interface-for-drac).

Download the iDRAC firmware and enable a TFTP server (google for it, or get one from solarwinds: http://www.solarwinds.com/downloads/)

Run the following command:

racadm -r <idrac address> -u <user> -p <password> fwupdate -a <tftp server> -g -u

The update will start and take a few minutes.