There's no charge to switch out of S mode. On your PC running Windows 10 in S mode, open Settings > Update & Security > Activation. In the Switch to Windows 10 Home or Switch to Windows 10 Pro section, select Go to the Store. On the Switch out of S mode (or similar) page that appears in the Microsoft Store, select the Get button.
i can not tell you if there will be a charge for upgrading win 10-S but unless you do only apps from MS store can be installed.
