How do I reset WPA/WPA2 security setting to WPA3?

Only newer routers support WPA3. If you want that you may need to replace your router -- but WPA2 AES is fine and won't trigger the Apple weak security warning.

To eliminate the weak security warning, change the security settings on your router to WPA3 or to WPA2 AES. How you do that varies by router. If you don't have the instructions for your router, do an internet search for instructions for your router brand/model or if your router is supplied by your internet service provider, check their website or customer support. Also see: Recommended settings for Wi-Fi routers and access points - Apple Support

You would need to access your Wi-Fi router's admin panel and change it there if it actually supports it.

Consult your router's user guide and documentation for steps to access the control panel, and steps to change the security level.

You will most likely need to connect to your router with an ethernet cable from a computer directly to make the change.