You should be very careful clicking on links contained within your emails especially from an unknown source, but even from known & trusted sources. If someone tells you that you need to update something, then go to their website yourself by typing in the browsers URL search bar. Do not trust anything contained in the email especially the URL which is provided. Navigate the company's website to look up the information to confirm. There is too much phishing these days to just trust anything that is sent to you.
Since you cannot download or run it on your iMac, this really sounds like a fraudulent email which is trying to install a virus/malware on your computer, but since you aren't using Windows you got lucky and avoided the virus/malware (hopefully). I've never run an app on my computer to update my router's firmware. At most I've download the firmware file and then must log into the router and use the router's interface to update the router with the downloaded file. If this link contained a fraudulent firmware file from an unscrupulous source, then updating your router with it would give this person complete access to everything on your network.
You may be able to update the firmware of the Netgear router just by logging into the router and checking for new firmware updates. Or you may have to go to the Netgear support website and manually check for firmware updates for your particular router and download the new firmware before you can use the router's interface to locate the file on your iMac. Check the router's user guide on how to properly update the router's firmware.
It is best to configure your email client to only show the text part of the email and only enable the HTML version once you've decided the email is safe.
Always hover over any links to view the actual address URL they will be using, but even this isn't always a good indicator since there are ways to hide the unscrupulous part of a URL so the URL address appears legitimate. This is good advice for browsing websites as well, but some browsers disable the feature of showing the URL in a link so this feature must be manually enabled.