There are no user extensions in Windows Explorer. "Extensions" are generally handled by Context settings.
There are generally many (over 300 on my system) context menu entries. If you are having problems, you need to review Contexts (screen shot below) that may be affecting your system.
Windows Explorer is not a Windows "App" (Settings, Apps) that you can repair or reset.
You may be able to just close all Windows Explorer occurrences, restart and see if your problem has been resolved.
If not, try the initial repairs for Windows 10. Open cmd.exe with Run as Administrator and run (in order):
dism.exe /online /cleanup-image /StartComponentCleanup
dism.exe /online /cleanup-image /restorehealth
Restart and test.
Below is a screen shot of managing contexts with ShellExView (Nirsoft). As noted, over 300 items and quite complex to view and understand the implications.