Open your file browser, look at the top left and click "view". Now, click on "options" to the right.
After this, click on "view" in the middle. Now, search for the option "Hide extensions for known file types", untick this.
Now, you should be able to see every file extension. You're able to change the file extension now, by simply renaming the file.
After you do this, it will give you a warning message, just click ok.