How do I change the security setting ?

Check your WiFi Router / Modem for --> Recommended settings for Wi-Fi routers and access points ...

Consider more secure wifi configuration.

Change your router’s WiFi to use WPA2-AES (CCMP) with no WPA or TKIP. Updating this setting ensures that your devices always connect in one encryption mode

1. TKIP is obsolete and not considered secure. Choose WPA2, the latest encryption standard, with AES encryption for optimal security
2. If your router doesn’t specify TKIP or AES, the WPA2 option probably uses AES

See the pics below

Perhaps Apple is telling you that WPA2 AES or WPA3 is a lot stronger than WPA2 TKIP. Access your modem-router and change your password type (if you have problems about that procedure, contact your modem-router manufacturer's and ask them).

Regards

Giulio

Apple only knows how to understand the level of encryption on the link without actually knowing what kind of security is being employed on the network, such as MAC address white lists, VPNs, and device security. So, all your phone can do is to imagine that your security is weak based upon the WiFi encryption and complain about it. Assuming that it is your router, you need to assess whether your security is suitable to your requirements or not. You may have devices in your network that are only capable of WEP encryption, so you may need to rely on MAC address blocking to stay safe. The bottom line is that you are the manager of your own network.