How do I update subscriber information through an import?
During the import process, on the "Map Subscriber Info" step, you can select if you would like to add only new subscribers to your list or update the existing subscribers too.
Selecting "Add new and update existing" allows you to edit the data of your existing subscribers in bulk. You can use this function to add new tags or update custom fields and subscribers' names. There are a lot of different ways that updating your subscribers' information can help you continue to target them and curate your messages towards their interests. We've included some examples below of how you can use this.
Updating Subscribers' Names
It's possible that you didn't initially collect your subscribers' names, whether it be because a name field wasn't included on the form or it wasn't required and subscribers neglected to submit their names. This poses a problem if you would like to personalize your messages and greet the subscribers. Personalized messages help draw attention to your message and ultimately help you build a rapport with your subscribers, which would lead to increased engagement. If the subscriber did not submit their name, then it will display as "Name Unknown" on the Manage Subscribers page. 
When you send a message using the {!firstname_fix} personalization, there will be a blank space, so rather than seeing "Hello Jane," you would just see "Hello." To make your messages more personal, you could update all of the existing subscribers without names and use a moniker like "Friend."
This way, when you send a message using the first name personalization, the greeting will appear as "Hello Friend," rather than simply "Hello."
Updating Custom Fields
Similar to updating your subscriber's names, you can update your subscribers' custom fields using the import tool. Custom fields are a great way to collect additional information on your subscribers, and send them targeted messages based on their preferences.
If you re-import and update existing subscribers, you can update your subscribers' custom field data. For example, if you are collecting your subscribers' favorite color, you can update your subscribers' information to include this new custom field information. Please note that custom fields can be overwritten or appended but cannot be removed through an import.
Updating Tags
Another way that you can use the import tool to update your existing subscribers would be to apply new tags to subscribers. Tags are keywords or phrases that you can use to group subscribers together and trigger specific campaign messages. You can also use tags to create a segment of subscribers and send a broadcast to that segment.
If you're connected to an eCommerce service, you might be considering tagging subscribers based on the product that they purchased so that you can send information about the product. Some API integrations do support tags, like Shopify, but not all integrations can add tags to existing subscribers in your list, which could be a problem if a subscriber purchases more than one product.
If you can export the customer information from your database, then you can re-import the subscribers and update existing subscribers with a tag related to the product they purchased. For example, if they purchased an eBook from you, you could tag them with "ebook." The tag will be added with the existing tags and will not replace any other tags, so you can have as many tags as you need.