Wi-Fi password does not work. Been had the same one for 15 yrs

See -> Recommended settings for Wi-Fi routers and access points

Recommended settings for Wi-Fi routers and access points - Apple Support

The Wi-Fi network name, or SSID (service set identifier), is the name your network.

And every Wi-Fi network name has a Password.

So sign in in the router and check.

iPhone XS, iOS 15

You can update your iPhone to iOS 16.2, you can check by reading this Apple article:

iPhone models compatible with iOS 12 -> iOS 16.

iPhone models compatible with iOS 16 – Apple Support (UK)

Go to Settings -> General -> Software Update -> Download and Install.

(on iPhone with iOS 16.0.3 & later) 

If you need help with your Wi‑Fi password go to -> Settings -> Wi-Fi -> tap i in the circle -> scroll to & tap on Password -> tap Passcode.