To add/remove or change an alternative login method on your D&D Beyond account, you will need to log in to your account with any method that is attached to your account and visit your 'My Account ' page.
From this page, you can link or change your Google, Twitch, or AppleID. To completely remove your Twitch login method you can select the ' Unlink Twitch Account '. Currently, Google or AppleID are unable to be removed completely without completely deleting the account. If you need to free up these login methods you can either change them to an unused Gmail / AppleID address or Contact Support to have the account deleted.