how do I secure my wifi network?

A "weak security" message is telling you your router is set to use older network security settings. You are not seeing this because your security has changed on your network, just that the new system software version on your Apple device is now programmed to analyze your wi-fi security and report it to you.

See this support article about recommended settings for Wi-Fi routers and access points

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