How do I change wpa.3 to Wpa 3.4

That's hard to say, my router doesn't have that setting either.

I've had this route for almost 3 years, so I'm not going to replace it.

See this Apple article: Recommended settings for Wi-Fi routers and access points

https://support.apple.com/en-us/HT202068

It depends on the router you have whether you can change it or not.

How do you know whether or not you have the correct router to be able to do that?

Look in the instruction manual for your router.