You can update a contact data field to have the same value for every contact in a segment or list at once. This lets you quickly create dynamic content for members of a given segment or list, or clean up your data where there are inconsistencies or outdated information.
Bulk update contacts in a segment or list
Go to Audience > Lists or Audience > Segments.
Select the check box for the list or segment you want to update the contacts in.
Expand the MORE ACTIONS drop-down menu, and select Bulk update contacts.
Expand the Data field to update drop-down menu, and choose the data field you want to update.
In the Update data field to box, enter a new value for the data field.
Select UPDATE DATA FIELD.