[khusamov]

var result = inspector.getEl().select("iframe");
var iframe = result.elements[0];
iframe.set({src: "newsrc"});

http://docs.sencha.com/ext-js/4-1/#!...-method-select
http://docs.sencha.com/ext-js/4-1/#!...ent-method-set
http://docs.sencha.com/ext-js/4-1/#!...mpositeElement