The sentence is correct as is.
This is because prepared is actually an adjective: http://www.merriam-webster.com/dictionary/prepared. If you want to get specific, in this scenario it is used as a subject complement; you can rearrange the words "prepared you are" to "you are prepared."
In short, you don't have to worry about tense in this example; though prepared ends with ed, it is not a conjugated verb. Some other examples of past participles that are used as adjectives are bored, interested, and frightened. However, if you wanted to change the sentence to use prepare as a verb, you could change the sentence to "The more you prepare for your exam, the better off you'll be."