It is very hard to convert these days. If you have "Prospective student" in your tourist than only it is possble, otherwise don't screwed up on changing status. Apply to school and after getting I-20 go back to nepal & get visa.. this is the easy process. Even if you change your status here in F-1 you need to get visa if you go back home for vacation. So, why not to go right now and get F-1 visa from Nepal. It would be vey easy to get F1 from nepal for Master's student.