Wi-Fi says it has a weak security . What should I do

This message is telling you that your router is not using the latest security settings. You can check your router's manual or check the make and model on the internet to find instructions to change settings. In some cases the router is part of the equipment provided by the cellular carrier. In this case, you should contact the provider. These are the recommended settings...

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

The weak security warning will be displayed on Apple devices if your router wi-fi is not set to WPA2 AES or to WPA3. How you change your router settings varies by router brand/model. If you don't have instructions, do an internet search for your router brand/model or if your router is supplied by your internet service provider, check their support website.