how do I change my T-Mobile wifi security to WPA3?

You do it through the phone app. Launch the app and then touch Network on the bottom.

The default network that T-Mobile creates, however, is a WPA2/WPA3 network, so unless you changed it, you already have it enabled. If you did change it, or it was modified unintentionally, follow the instructions below and then scroll to WPA version and make sure you've enabled WpA2/WPA3 for the network you're using.

You may be seeing the notice you mention because your network is Hidden. Scroll to the bottom of the same page and slide the Hidden switch to to the off position (all the way to the left.) Click Save and then see if that doesn't affect the message you're receiving.

Macs in particular will tell you that a Hidden network is insecure. Hidden is kind of a misnomer since the network is not really invisible. Any good hacker with a $1 WiFi finder can find a hidden network in under a minute. In other words, it's a false sense of security to think that hiding a network will make it more secure.

Hope this helps.