Windows 10 S is a simplified version of Windows 10 Pro, locked down to Windows Store apps only. Microsoft released a new FAQ page on the Microsoft website today. It includes some of the basic details surrounding W10S. Microsoft Edge is the default web browser on Microsoft 10 S. The default search provider in Microsoft Edge and Internet Explorer cannot be changed. Currently, only Edge and Opera Mini are available through the Windows Store, meaning there s little to choose from so this is for now a non-issue.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-10-s-wont-let-you-change-the-default-browser-meaning-youre-stuck-with-edge/