How do you revise for Spanish exams?

A key thing to remember is that with languages it's impossible to cram everything you know in one go because it's a long process of building up your knowledge of vocabulary and grammar in order to give you the best chance to express yourself both in writing and orally. I recommend looking at news websites such as El País or BBC Mundo to have a better knowledge of how sentences are constructed or reading more fun articles on Spanish Buzzfeed to learn new vocabulary as you never know what may come up! There are a wealth of great resources for Spanish as much of South America is Spanish speaking as well as vast parts of the US population. 

Closer to the exam, it's about knowing the subjects that may come up and knowing the key vocabulary for the topics as well as the key controversial issues and both sides of the argument. Listening to Spanish radio or news can only ever help you!