xmlObject - set value of element with a particular attribute

Hi,

In a xmlObject I try to set the value of xml element with a particular attribute butit doesn't work

//get value of element with a particular attribute
var myValue = xmlObject.ELEMENTS.ELEMENT.(@category == "myCategory"))// works fine
//set value of element with a particular attribute
var myValue = "foo"
xmlObject.ELEMENTS.ELEMENT.(@category == "myCategory"))= myValue // doesn't work

Thanks for your help

Regards