As you are reading this post I believe you already know how to create external entity CRUD and specific finder methods and you already have created multiple methods.
1) Setting default method for external entity created in SPD. Open SPD and edit your target (finder/updater/creator/delete) method/operation. Check the “Use this operation as the default finder/update … operation” and save it. This is always checked when you are creating new operation in SharePoint Designer.
2) Setting default method for external entity created in Visual Studio 2010.
Open the .bdcm file for your BCS project. Set Default =”True” for your target method.
Make sure that you have only one default method per operation.
Already created external list will not adopt this change. You have to create new external list to get this change in action.