1. Click the drop down arrow in the top right hand corner of the Web Browser.

2. Select Admin

3. Scroll down to the Developer Tools section and choose Studio

4. Choose Leads or Contacts

5. Click on the field that says "Lead Source"

6. Choose "Edit" to modify the drop down list.  

7. In "Item Name" input the name of the field in lowercase and with underscores instead of any spaces.  

8. Next, you'll input the "Display Name".  This is the name user's will see when viewing the field through the CRM.

9. Click "Add" to have the new entry appear in the list.

10.  If you need to add more drop down values to the list repeat steps 7-9, otherwise click on "Save" to finalize the list.

11.  Hit "Save" one more time on the next screen and then you're done.  The new entry will now appear in the drop down list.