In Windows 10, MovieMaker is no more. Win10 comes with the app called Photos. Open the video clip there to edit it (it should be what happens by default if you double-click the video). Unfortunately, rotate is not an option for editing the video, BUT, if you click the Edit/Create button, then Create a video with text - you are now in something that looks a little like MovieMaker and there is a tool for rotate.
Hope that helps.
OR
with the video on your phone, you can open it in Google Photos (or Samsung Gallery), tap the edit button and Rotate.