If you go to the device settings and scroll down to the credentials section, you should be able to find the Credentials for HPE 3PAR. If you break the inheritance it should allow you to make your own settings.
This option should be available on the device settings. Make sure you are running the lattes PRTG version. If you are still not able to find this option, we would need to have a closer look at your system. To do this, it would be a good idea to open a ticket and make reference to this KB article in order to further investigate it.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.