After you purchased an upgrade from your Single license to Studio, you don't need to upload the new theme files into WordPress.
In Semplice dashboard navigate to "Settings" > "License" and deactivate your Single key. In the license type dropdown select "Studio License (Upgrade from Single)" then paste your new license key. Click Save.
After saving your changes at the top of the page you will see the banner that will ask you to upgrade your edition. Click on the link. This link will lead you to WordPress > "Themes" where you can manage your themes. On the Semplice theme thumbnail, you will see the "Update Now" button. Click it to upgrade your version. After the upgrade is complete, you can launch Semplice.
Please note, the upgrade process will not affect your content. This process will only unlock all the features for you.