Need advice on how to make S9's WiFi forget my previous router password...

From the Wi-Fi Settings area (you can access that by long-pressing the WiFi toggle in your notification bar), you have a couple of options. You can just delete the WiFi connection (long press, 'Forget network') and when you re-add it it should ask for credentials again. Or, you can use the same connection and just long-press it, this time select 'Manage network settings' and edit the Security section with the updated credentials.
 

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