This guide explains how to change the file extension in Windows 11. It also clarifies the distinction between file extensions and file types. Change a File Extension the Easy Way The simplest way ...

But the Windows 11 Settings app uses a more granular system that sets default apps one file extension at a time.

If you cannot change the Default Program Extension, or the option to change it is grayed out in Windows 11/10, follow this tutorial to fix the problem.