Universe uses your Apple ID to login and register. We don't have a username/password system.
Universe uses your Apple ID and iCloud account to login and manage your websites. To manage your sites on another iOS or macOS device, make sure you're logged into the same Apple account.
Since we do not use a traditional username/password method for logging into the app, there is no password for you to change or reset. If you need to change the password on your Apple ID, check out the article attached below.
Please note that changing an Apple ID password will change this on all of your devices, and we have no way to change it back.
Check out this article to change your Apple ID Password.