GS724Tv3 on old firmware

I have the above mentioned switch which for the last few years has been used as a dumb switch in default configuration. It's been solid since the original install so i've never bothered doing anything with the configuration. I've recently had some network issues; slow speed with wired devices so i started looking for solutions. the switch is running boot version 5.0.3.2 software version 4.2.1.8 which is well out of date. Under the maintenance section of the gui there is no option to upgrade firmware and the netgear discovery tool doesn't find it. Any suggestions how i can bring the firmware up to date? Any help would be much appreciated TIA