Short answer
HubSpot's KB: HubSpot sets Original and Latest Traffic Source from tracked activity. Latest may update many times. You can edit values manually, or with a workflow's Edit record action on Professional or Enterprise.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Editing original source by hand overwrites what tracking recorded, so do it sparingly.
3. Related
Source on deals and Other campaigns.