How to change security settings on the router and Wi-Fi I have a week security

This will depend on your router.

Normally you will log onto a web page of your router (you might need to know the password for this - often supplied on a card with the router, or printed on a label - or even a factory default password) and change the settings in there.

For full details specific to your router, you will need to go to the manufacturers website, and look at FAQ, help or documentation, or ask on the manufacturers forum.

One other point here.

You don't need to panick - the security is no weaker now than before you installed IOS14 - it is just that the phone is telling you about it now.

You can take it steady to find out how to fix it.