Got a message on my Apple iPad that my Wi-Fi has weak security. Where in Settings do I change this. I couldn’t read my email on my iPad, but my iPhone is good.

You have to change the security settings on your router. You will need the admin user and password for the router. See the information in this support document. Recommended settings for Wi-Fi routers and access points - Apple Support

Additionally, if you have an older router with outdated security options, you can talk to your internet provider and ask if they have any models that have the newer security options.

Just so you know, you've always had weak security - you just didn't know about it. The new iOS version now alerts you to it. You should be ok (if a new router is not an option) as long as you use a very long and complicated password.