It's very simple! Please read this subsection of the documentation for developers if you are using the BigML API. If you are working with the Dashboard go to any of the main tabs (sources, datasets, models/ensembles/evaluations, clusters, anomalies, associations, predictions or tasks) and press x on the project selector:
As you can see below, BigML displays All resources you have created, independently of the project to which they are assigned:
