Updates a custom field label (see Create a custom field above for more information on labels). Note that the key will change but the name remains the same when the label is updated.

Warning: An update to a custom field will break all of the liquid personalization tags in emails that reference it - e.g. if you update a Zip_Code custom field to Post_Code, all liquid tags referencing {{ subscriber.Zip_Code }} would no longer work and need to be replaced with {{ subscriber.Post_Code }}.