I have an elder brother he is all intelligent and everything... he is having trouble with clearing his degree exams... he is 26 years old... he says that he cannot study things that don't interest him... I don't know what to do... he easily gets tensed.

He is 26, and you cannot force him into something which he is not interested in. But let him know that you care for him and you are worried about his degree. Tell him like, "You know it by yourself, that it will just be a matter of time to clear the degree and get over it. I dont have to tell you at all"
Make sure that you are not advising him, but rather, you care for him. He will definitely take his responsibility.

Can you study with him? Ask him questions from the textbook or his notes? Help him make flashcards to study factoid kinds of things? If he sees you are working hard, maybe he will work hard too.