If your form is submitting to a SharePoint form library, you can simply reopen the existing record (XML) and resubmit it. If your SharePoint library submit is configured correctly, this will overwrite the XML in the form library.
You could create a "dashboard" form, which asks users to select an item from the dropdown, loads the data from that XML, and allows the user to overwrite. That's a lot more difficult and probably requires code.
So if the first option sounds reasonable, try it out before venturing into the second alternative, because it will be much more difficult to put together.
Good luck!