To update fields (both pre-existing and new) to existing contacts, you'll need to create a CSV file using your spreadsheet app of choice (Excel, Google Sheets, Numbers, etc.). The first column should include the email addresses in your list (Fig 1). You can then add remaining columns to be updated or added. 

Fig 1

Note: Fields that are not being updated do not need to be included in the spreadsheet. 

In the Navigation Menu on the left, click Contacts -> Import Contacts (Fig 2).

Fig 2

Select the database you want to import the Contacts to (Fig 3). 

Fig 3

Before selecting the "Next" button, drag your CSV file over to the Import Details box on the right (Fig 4). If is has headers and you want to add contacts to autoresponders, check the checkboxes. 

Now, go click the Next button. 

Fig 4

On the next page, make any changes to the fields mapping necessary. If it's not clicked, be sure to click the "Overwrite" checkbox in the second column (Fig 5). 

Fig 5

 After clicking the "Next" button, you'll be given the option to add the new imported contacts to a separate list (Fig 6). Click the "Next" button again. 

Fig 6

On the Permission-Based List screen, type in your initials, click to proceed, and click on "Start Importing" or "Start Importing in the Background" to import your contacts. 

Fig 7

This will upload values into the fields 'City' and 'State' for all contacts.