How to edit node in xmlHttpObject

How to edit node in xmlHttpObject

I'm putting together an online xml editor using jquery for the interface and a php back-end. I have the php script send a snippet via ajax() to the web-page as an xmlHttpObject. This xml data is presented in editable form fields using jquery. What I'm having trouble with is taking the data from one of those form fields (textareas) and using it to change a particular node in the xmlHttpObject before sending that back to the php script. Any suggestions? Can I manipulate the xmlHttpObject or do I need to serialize the form fields and create a new xml object?