You can modify a contact's details in the Contacts tab.
First navigate to the Contacts tab from the top-left corner of the screen. Then find the contact you wish to edit and hover your cursor over this contacts row. Next, click on the small icon of a person on the right side of the row, see the picture below. Through this button you can edit the details of a contact.
Make the desired changes and click Save. Note that editing the contact information through this view will not edit them in the feedback you have already collected. The only way to edit contact details in surveys where you already have feedback is to contact Zeffi support support@zef.fi