Hello @pvdm,
There is no direct relation between the MyUPC credentials and your Horizon box credentials. If they were the same at some point it is because you set it like that or because someone from support used the same credentials for configuration at some point.
The password of the Horizon box can be changed via the GUI which is generally found at http://192.168.0.1 under the System tab.
If you forgot the admin password you have set, I believe the only solution is to perform a factory reset via the TV interface but there you might want to call support to confirm first.
The password set by UPC can be generally found under the Horizon box on a sticker.