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 JavascriptBut if you are looking to dissect it you may find it difficult since the code is obfuscated.
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