The "Set Data Source Credentials" is only used for Information Links that require authentication while opening analysis and cannot be used as an independent job to update the username and password for the data source. Here is an extract from the manual:

----------------------------------------

Set Data Source Credentials

If you are going to open an analysis or information link requiring data source authentication, you can specify the credentials to use with this task. This task should be performed before any Open Analysis from Library or Replace Data Table with Information Link tasks. In order to secure the login credentials, you must encrypt them using an X.509 Certificate. Note that you need to add one Set Data Source Credentials task for each data source that requires authentication.

----------------------------------------

You can achieve this via the API. See the following reference.

https://docs.tibco.com/pub/spotfire_server/10.10.0/doc/api/TIB_sfire_server_Information_Services_API_Reference/index.html