This is a screen capture of the Entities editing page. In this image, a data query entity has been selected from the list of entities. This selection opens the data query entity editor, which is divided into the following tabs: General, Configuration, Presentation, Attributes, and Dataset. 

In this image, the Configuration tab is selected. The partial view of this page showqs the Backend Mapping section expanded to reveal the Data Service and Table fields. Directly beneath these fields is  the Primary Key field. Next to the value that populates this field is the Edit option.