Importing Contacts - Overwriting

Importing Contacts - Overwriting

I am trying to batch update contacts with one of a drop down field by creating a csv file with just account name, first name, last name and the drop down field I am trying to change. Before importing, I choose the overwrite option, however instead of overwriting the existing contact, it creates a completely new one with just those 4 fields specified. 

How do I overwrite existing contacts with just one field from a csv file?

Thanks