I expect you need to log into your account on each channel's website. They probably have a "forgot password" option.
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Re: Not able to log into OAN Live and Discovery Plus
@TonyE, if you subscribe through Roku you never need to enter any credentials. Your subscription is only good on Roku devices and only on Roku devices linked to the one account where the subscription was initiated. There is no "resetting credentials" as it doesn't make any sense. The channel/app should check with Roku's servers when it starts to see if you have a subscription through Roku.
It's not exactly clear what you're trying to do, but if you're trying to use your Roku-based subscription on a non-Roku device or on a Roku device that's not linked to the proper Roku account it's not possible.
To answer your last question, if it was me that's exactly what I'd do.
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