Your router is the one device in the house that every other device talks through. That makes it the most interesting box on your network to somebody who wants in—and the one people are least likely to have touched since the day it was plugged in.
Why Routers Specifically
A router is always powered on, always reachable, and almost never watched. There's no antivirus icon in the corner telling you something looks wrong.
That's why compromised home routers get quietly recruited into botnets, used to relay traffic for other people's crimes, or pointed at name servers that hand back the wrong address for a site you meant to visit. The attacker usually doesn't care about you at all. They want the position your router occupies.
How to Actually Check
Log into your router's admin interface—a web address or the manufacturer's app—and look for a menu called Firmware, System, or Maintenance. Most modern routers will tell you the installed version and offer to check for a newer one.
While you're in there, turn on automatic updates if the option exists. We'd rather a router patch itself at 3 a.m. than wait for someone to remember. If yours only supports manual updates, put a reminder on the calendar a couple times a year and leave it undisturbed while the update runs.
If we supplied and installed your router, this is one less thing on your list. It left our hands on current firmware, and keeping it current is part of what supporting it means—you don't have to remember to check. If you're not sure whether the router in your house is one of ours, ask us next time you get in touch.
Two things to do on the same visit: change the admin password if it's still the one printed on the sticker, and turn off remote administration unless you know you need it. An admin page reachable from the open Internet is a standing invitation.
When It's Time to Replace It
Firmware updates only help while the manufacturer is still writing them. Once a model reaches end of support, no amount of checking will produce a fix, and the known holes in it stay open permanently.
Search your model number plus "end of support" and see what the manufacturer says. If the last release was years ago, or the vendor no longer lists the model at all, it's done—however well it seems to be working. We see plenty of routers out here that are a decade old and still passing traffic, which is exactly what makes them dangerous.
Check your firmware this month. If the answer is "there is no newer firmware, ever," start shopping.