In order to "forget" your preferred WiFi settings and reconfigure for your new username or password on your Apple device, you will first need to go into your settings.
From there, you will need to select the Wi-Fi options in the left side-bar menu. At the top of the list of Networks, you will see the one that you are currently attached to. Out to the right of that Network name, you will see a blue circular icon with an "i" in the middle.
Click on the icon and select "Forget this Network" at the top.
Now, you can go back to your list of available networks and select the option for your new network connection.
When selecting a secured network, you will be prompted for some login information. Simply enter the same username and password that you use to sign into computers, and you will be able to utilize the new network connection.