If you activate your Premium plugin license from the Mailchimp for WP > Other page in your WP Admin, then the plugin will check for updates like any other WordPress plugin. So normally, you should not need to manually update the plugin to its latest version.

If however for some reason the automatic update is not available, you can manually update the plugin by taking the following steps.

First, log-in to your account area and download the latest version of the plugin.

Using WP Admin & a plugin

1. Install the Easy Theme and Plugin Upgrades plugin (free).
2. Navigate to WP Admin > Plugins > Add New > Upload.
3. Select “Yes” for the “Upgrade existing plugin?” option.
4. Browse to the location of the plugin ZIP file on your computer and press “Upload”

Using FTP

1. Unzip the downloaded ZIP file somewhere on your computer
2. Connect to your webserver over FTP and navigate to /wp-content/plugins/
3. Locate the plugin directory: mailchimp-for-wp-pro or mc4wp-premium.
4. Replace the entire directory with the files from the downloaded .zip file.

By following any of these two methods to update the plugin you will never lose any of your configuration settings.