How to update Wi-Fi network security

OOPS, my prior answer was wrong. Here is the CORRECT answer:

This is NOT an iPhone problem; it is a problem with the settings on your Wi-Fi router.

A "Weak Security" notification means that your Wi-Fi router is not configured correctly. There is a new feature in iOS 14 and later and Mac OS Big Sur (11) and later that checks and displays a message if your router is not using the most up-to-date security settings. Note that your router has always had weak security, you just didn’t know about it until now. 

See this Apple support article for how to configure it correctly—>Recommended settings for Wi-Fi routers and access points - Apple Support, specifically the section headed Security. And don't forget to configure your range extenders (if any) also!

After you change the router settings you should Forget the network in your phone settings and reconnect.

Note also that weak security does not interfere with using Wi-Fi, it is just a warning. Your Wi-Fi should still work normally, and, if it doesn’t, it is not because of the weak security message.

Your WiFi router may not be capable of providing the highest security options. If you could provide the make and model number of the product, we'll do some checking on the device.

Apple's recommendations are as follows:

 Set to WPA3 Personal for better security

 Set to WPA2/WPA3 Transitional for compatibility with older devices

The security setting defines the type of authentication and encryption used by your router, and the level of privacy protection for data transmitted over its network. Whichever setting you choose, always set a strong password for joining the network.

• WPA3 Personal is the newest, most secure protocol currently available for Wi-Fi devices. It works with all devices that support Wi-Fi 6 (802.11ax), and some older devices. 
• WPA2/WPA3 Transitional is a mixed mode that uses WPA3 Personal with devices that support that protocol, while allowing older devices to use WPA2 Personal (AES) instead.
• WPA2 Personal (AES) is appropriate when you can't use one of the more secure modes. In that case, also choose AES as the encryption or cipher type, if available.