Perhaps there is some base intelligence innate to people, perhaps that cannot change, some people are more likely to understand quantum mechanics than other people, for example.
Your question is misguided. IQ is a social constrcution, a social interpretation of intelligence. There has been a LOT of research on it in sociology and other social sciences and most say that IQ and IQ tests are more about 'cultural competence' than intelligence.
You may have heard that black people do worse off in IQ tests statistically, the same applies to a few other minority groups too. This is not necessarily because they are dumb or are naturally less intelligent (I abhore such explanations). But rather, because the people who make the tests are predominantly white and middle class, giving cultural presuppositions that people who aren't white or middle class may not understand.
But to change your question, how do you want to be more intelligent? Here are a few things you can study:
Logic - the science of arguments
Various branches of mathematics - mathematics unveils the structure of the world in an abstract way, it makes you think abstractly - mathematics leads to the natural sciences and is much more fundamental than that.
Social sciences - developments in social science reveal a lot about what human beings just took for granted and places it in flux. issues that open your mind in social science involve:
'Nature or nurture debate' (psychology)
'Homo economicus and rational choice theory' (Economics)
'The social construction of the norms we take for granted' (Sociology)