In your ‘change’ event handler code, you would basically be using regular html/javascript techniques. There isn’t much different while using CAF at this point.
For example, something like this as the value of the ‘Client-Side Events > Change’ property of the dropdown control:
// the 'this' variable would be the dropdown element that changed, you can get the current value from that.
var value = this.value;
//get the iframe by the id, and change the src property of the iframe to the new value
var frameId = '#{caf:cid("myframe")}';
var iframe = document.getElementById(frameId);
iframe.src = value;
#webMethods#webMethods-BPMS#MWS-CAF-Task-Engine