You can reset the password for user "yodeck" through the configuration item named "Linux yodeck user password". Just enter a password there, it will be encrypted properly and written to the system. We only save encrypted (hashed) passwords, so we never know your actual passwords.
IMPORTANT: Changing the password (or SSH keys) for the "root" user is not recommended. This allows us to provide direct support in case you need it. But if you wish to, you can indeed change it; the Player won't be affected in any way.