From Microsoft’s FAQ about Windows 10 S:
Yes, Microsoft Edge is the default web browser on Microsoft 10 S. You are able to download another browser that might be available from the Windows Store, but Microsoft Edge will remain the default if, for example, you open an .htm file. Additionally, the default search provider in Microsoft Edge and Internet Explorer cannot be changed.
Braindead. Edge is buggy and messy, Bing is garbage. Not being able to change default applications is one of the many reasons using iOS is so grating and cumbersome, and Microsoft copying that behaviour is really, really dumb.