How to do a bulk upload for the Extensible field in Address section of Customer site

Summary:

We have created extended attributes from Application Composer in the Address object. Now we want to update that field value to multiple customer sites. Hence we are looking for an option to do bulk upload. But we are not finding the appropriate fields to achieve the same.

Content (required):

Customer Channel field below is an extension field added to Address object in Application Composer. How should we do a bulk upload in this field.

Version (include the version you are using, if applicable):

23C

Code Snippet (add any code snippets that support your topic, if applicable):