In my opinion, "being smart" is not exactly the same thing as "having much knowledge". Given a problem to solve, knowledge helps you make an informed decision,

In my opinion, "being smart" is not exactly the same thing as "having much knowledge". Given a problem to solve, knowledge helps you make an informed decision, while smartness helps you find a hack. Of course, they're not totally unrelated. I mention this because I believe it'd be helpful for you to consider which of the two you want to improve in more. Knowledge can come from books and courses, like many people are recommending. I wish I knew how to become smart (perhaps if I was smart, I'd find a hack for it...), but I suspect it might help to get into the habit of questioning everything, and practicing lateral thinking. Perhaps it would also help to try and find abstract similarities between situations from very different contexts - often new insights can come from combining knowledge from different fields, and this will probably help you get used to seeing things in different contexts. Considering the (no doubt intentional) vagueness of the question, I'm guessing you'd be interested in both. I think a good way to improve both at the same time is to often read, and then try and find something to read that says the opposite. That way you get a lot of knowledge, often from various sides, and it invites you to think about it more and question the assumptions. I hope that helps.