Rename the Lead Source field label in Salesforce

The Lead Source field on the Leads controls what field label is displayed on Leads, Contacts and Opportunities, so to update the Lead source field on any of the 3 objects:

In Classic:

1. Setup
2. Customize
3. Tab Names and Labels
4. Rename Tab and Labels
5. Click "Edit" by the Leads Object
6. Click Next - Search for Lead Source
7. Scroll to the Lead Source field in Standard fields
8. Update the Singular and Plural labels.

In the Lightning Experience:

1. Click the gear button on the upper right side.
2. Select Setup - On the Quick Find box, search for Rename Tabs and Labels
3. Select Rename Tabs and Labels which is under the User Interface
4. Search for Leads
5. Click Edit beside the Leads
6. Click Next - Search for Lead Source
7. Set the new name for Lead Source
8. Set the Plural for this new name
9. Click Save