Mass update Lead Source

I wanted to mass update the Lead Source for a number of contacts in the database.
I have tried two methods:

  1. Using csv file
    Using this interface from Admin > Data Cleanup > Modify Existing Records, I uploaded a csv file with updated values into Infusionsoft and matched the existing records based on InfusionsoftID. I noticed other fields have their value updated, but Lead Source field remained unchanged.

  2. Using a Campaign
    In a Campaign, I used Set Field Value Process to update Lead Source value. I have the “Overwrite existing field value” checkbox ticked. I assigned a trigger TAG to send contacts to this campaign. Again, the Lead Source field value wasn’t updated.

Is there a way to mass update Lead Source for existing contacts rather than manually updating? I have more than a thousand records needed to be updated.