Try this.

Select the contacts that you want the leadsource field changed. Export the ContactID, Contact Name (not necessary but helps to view) and Leadsource to a CSV file. Open the CSV file in excel and change the Leadsource value to what you want, save the file (as CSV again) then import the file back in to your InfusionCRM. Make sure you select the checkbox for "I want to update existing records" when you import back in to Infusion so the records are updated and not duplicated.