Microsft Windows 10 in S mode is specially created for security and performance, exclusively for running apps by the Microsoft Store. If the user want to get an app install that is not in the Microsoft Store, The user will need to switch out of S mode. Switching of S mode is one and the only way. If the user make the switch, The user will not be able to go back to Microsoft Windows 10 in S mode. There are no charge to switch out from of S mode for the users.

On your computer running Windows 10 in S mode, tap on Settings then tap on Update & Security and finally select Activation.
If in terms to Switch to Windows 10 Home or Switch to Microsoft Windows 10 Pro section, Tap on Go to the Store. (If The user also sees an “Upgrade your edition of Microsoft Windows” section, be careful/aware not to tap the “Go to the Store” link that is provided there.)
After the Switch out of S mode (or familiar) page that comes up in the Microsoft Store, tap the Get button. After the user see a confirmation on the page, The user will be able to install the apps from outside the Microsoft Store.