How do I change my router. I keep getting a message that my WiFi is low security???

Look in your router’s user manual (Google)

You need the IP address, the Admin name snd Admin password

These may be on the back label

Perhaps this link will help...

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

You may not need to actually have to replace your router, but most likely, its wireless security setting to eliminate getting this warning message. I suggest you review your router's owner guide to see how to configure your router's wireless network to use WPA2 (or higher) security.

No panic. Remember it is no weaker than it was when you were on iOS13, it’s just that iOS14 introduced the warning.

The Weakness will not cause slow internet. That is down mainly to the provider, the cable, and possibly to the age of the router,