I'm not 100% certain I understand the problem, but I think you mean the Roku isn't allowing you to enter the new password into a previously joined network. The fix for that is to go into the Roku settings menu, under System/Advanced system settings, and perform a network connection reset. This removes all stored network credentials (nothing stored within the channels themselves). After that, you should be able to select and enter the password for your WiFi access point.
Dan
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