My WiFi on my iPhone says weak security

A "Weak Security" notification means that your Wi-Fi router is not configured correctly. There is a new feature in iOS 14 that checks and displays a message if your router is not using the most up-to-date security. Note that your router has always had weak security, you just didn’t know about it until now. This is NOT an iPhone problem; it is a problem in your router settings.

See this Apple support article for how to configure it correctly—>Recommended settings for Wi-Fi routers and access points - Apple Support, specifically the section headed Security.

It isn't telling you that you had a weak password, it is telling you your security setting on your router is not secure. You need to log into your router and change it there, making sure to change both bands and any extenders. After you do that restart your router and on your iPhone forget the network and connect to it again.