How to edit an external activity?

I have a set of contacts I've uploaded external activities to the wrong campaign for. How can I either delete the activities so I can re-upload into the correct campaign, or edit the existing activities? I don't see any options in the API documentation nor within the Eloqua UI.

To be clear: I'm not trying to edit external activities themselves, but rather the campaign response association between the contact and the activity. The contacts have been associated to an activity in the wrong campaign, thus incorrectly inflating their lead scores.