Upon uploading the dashboard to the Power BI service, the data source should be stored within a "Semantic model" that is accessible from the Home page list. Users are granted the capability to adjust the settings of this model as illustrated below:

It is allowed to edit the credentials and cloud connection settings in the sections of "Gateway and cloud connections" and "Data source credentials".

Based on my experience, refreshing the SharePoint data source can be somewhat time-consuming in Power BI Service. It might be beneficial to consider exploring alternative methods, such as establishing a streamlined ETL pipeline for transferring data from SharePoint to a database and enabling a more straightforward connection for Power BI.