You build a Desktop from a data source, how do you point that desktop to new data source

A little vague, but I'll give it go. You either create a new query or you edit your query and click on the "Source" step and then edit the formula for that step.  You can also use the Advanced Editor for this. I have done this with Excel files moving them from local to a network drive for instance.

You can edit the source statement as discussed. I've done this with OData feeds as well. However, be careful that the data schemas match or steps that remove columns or create new columns based on another column will error out.