Update existing subscribers from a file
To make bulk updating of subscriber information really easy, you can make changes outside your account, then import them as a file.
The unique identifier for all subscribers is their email address. That means you can export your list of active subscribers, add new or changed information to the file, then re-import it to the same subscriber list.
This method can be used to update existing custom fields, where you might already have information recorded for some subscribers.
NOTE: If a value is left empty in the file you import, instead of overwriting existing values with nothing, the original values in the subscriber list will be kept.
You can also add new subscriber data to the exported list and create a custom field for it during the file import. For example, a new numeric field for subscriber ages:
After the file has been imported you'll see a file import report including information on how many existing subscribers were updated and how many, if any, were unchanged:
The file import report doesn't make reference to the custom fields you've updated, or just created, it only displays a sample of subscribers who've been updated and lists the total below that. For example: Showing 3 of 14,209.
To check if your subscribers have been updated as expected, you can create a segment based on the custom field and check to see how many people are in it after the file import.