The “Sync to Document” button updates the document with elements from your design library. For more details, click the “?” icon next to the button.
To update your library, select a symbol in the document and click the “Add” button in the section you’d like to upload it to in the Sketch plugin.
To add text styles, switch to the Text tab in the plugin, and click the “Add” button, then select the styles to upload. Similarly, to add shared layer styles, switch to the Shared Styles tab and click “Add”.
Let us know how this works for you, and if you run into any issues, feel free to shoot us a message at email@example.com.