A good tip which helped me in the past was to revise a course by writing an over-view (summary) of each section and then outlining areas I needed to improve in. Don't 'over-study' however, just write one page about each section of the course you are currently studying, and then in red ink decide what you need to re-learn.
You can do this simply by re-reading the topic which is troubling you just before bedtime each night. Just for 15 minutes, while making occasional notes. This can give your mind a chance to absorb some information at a time of night when you are relaxed - and your learning should improve.
Try not to panic as this will inhibit your ability to learn. Stay calm and put in the time to review your course, ask your teacher about the areas which you have never quite understood and read up on these issues before you near the exam time. You can never exactly anticipate what the exam will entail, but you can prepare and feel more confident and face the exam calmly.
For advanced students who are 'brushing up' (improving) their English, then please keep a notebook and list the words which give you continual problems. Practice your English frequently with English speakers and don't worry if there are still words which escape meaning for you. By keeping a list and increasing your vocabulary and pronunciation skills you will improve your already good English into excellent English.
For everyone who is worried about their exams the key is relaxing, taking a deep breath, and doing the review work of their courses to increase their knowledge base and confidence.