Assuming you are talking about downloading the "Javascript" ArcGIS API, you can find instructions for that in the API documentation:
http://help.arcgis.com/EN/webapi/javascript/arcgis/help/jshelp_start.htm > Concepts > Getting Started > Get the ArcGIS API for Javascript
But if you are looking to dissect it you may find it difficult since the code is obfuscated.