Thanks for the inquiry.
How are you subscribed to the Discovery+ channel? If you are subscribed through Roku, you will be unable to log into the channel outside of the Roku device. However, if you are subscribed to Discovery+ through the channel provider directly, you will need to contact the channel provider for further assistance regarding changing the account authentication information.
You can reach reach Discovery+ support here: https://help.discoveryplus.com/hc/en-us
Roku Community Moderator