Ensure that you have a backup of your configuration file by logging into the router at http://192.168.1.1 and clicking on Administration and then Settings Backup & Upgrade. Subsequently, you should take a screenshot of the vital settings in case the backup is incompatible with the newly installed firwmared.
Head to the Support Page http://support.netgear.com/ and locate the Netgear ProSafe device. Ensure that you have read the release notes for any possible compatibility issues.
Download the firmware and head back to http://192.168.1.1 and head back to the same Settings Backup & Upgrade page from before.
Make sure to select the firmware file that you downloaded prior. Click on upgrade and allow the ProSafe firewall to walk itself through the process. The device will reboot after the firmware is succesfully installed.
Ensure that your system successfully accepted the firmware by going back to the Admin page (192.168.1.1) and clicking on System Info to ensure proper firmware version.