Sometimes having the management software will not allow the firmware update utility to work try the following steps:
Hello,  I have the error "No UPS connected or unable to make connection"  The UPS is connected by USB.  I have RS232 or USB selected on the upgrade utility.  I am running UPS Local Node Manager, this is how I know the USB connection to the UPS is good.  Local node manager says under power source: Dell UPS Rack/Tower 2700W LV UPS LI RT 2700.  The device state is showing UPS fault.    Following information found here I am to update the firmware and then re-seat the battery followed by replacing the battery if the fault is not resolved.  I have re-seated and replaced the battery with a new battery.  The front panel of the UPS still shows the fault as well as the software after replacing the battery.  It seems the firmware upgrade is the only option to correct this error at this stage in troubleshooting.  However, the upgrade utility is not working.  Please assist.
Error on Dell UPS Firmware Upgrade Utility