You can find documentation by navigating to your Epicor server like this
epicor server]/EpicorERP/api/help/v2/ There you will find all of the different categories of APIs available, such as Business Objects, Reports, Libraries, Procedures, Etc. Here is one service exposed by the Business Object
epicor server]/EpicorERP/api/help/v2/odata/ This is a swagger page that goes very well into detail on how to interact with the API