Have chosen a field without thinking much I am pursuing Masters of Human Resource Management and will complete it within a few months. But it always bugs me that I have not chosen the right field. I went with what everyone was doing. I also think that this seat could have been gone to a deserving candidate who actually wanted to do this course. Such thoughts bother me. Can you please help me out with this, both the issues? I just want peace related to it.
Over the course of a lifetime a person makes many thousands of decisions, most of them with incomplete information. "It Seemed Like A Good Idea At The Time". Many of those decisions will lead to less than optimal results, so we learn to change and adapt. 

You need to give some thought, I guess, about what your goals are and your options. Use the information you now have to try to set a new path. It will take time and effort, of course, but there is little value in continuing on a path that no longer seems right or has value to you. Likely, finishing your current degree gives you more options than dropping out of it, but that is yours to judge. But you have various educational assets that you can employ to change directions. But you need to try to make the best decision you can, knowing what you know now. 

As for the other question, of denying your current seat to someone more deserving, it would only be an ethical issue had you set out at the time with the intention of denying that seat to another. That seems pretty unlikely. You did what you thought best. Others concurred with your decision. Now you have more information, and perhaps have changed and grown. Anyone who might have had your seat also had their own options and have, by now, made their own choices. You can't change that, nor should you worry about it. Someone else might be thinking the same way about their choices denying you an option that you might have had. But there is no global, universal, optimization scheme that sees the future and makes the objectively best decision for everyone. 

Do what you think best with the information you have at the time. If you choose a path, follow it faithfully, but not rigidly. Let change happen.