Short answer
HubSpot's KB: edit a value from the object index page, the record, or a segment; changes save automatically. Read-only properties such as Original source are set by HubSpot analytics or integrations and can't be edited. If a value won't save, the property may require a unique value.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
Copying values between contacts, companies and deals needs a workflow, not a manual edit.