There is no way of going back after confirming your answer.  Therefore, we warmly recommend that you review your answer before moving on.
Read how to do it wisely, without wasting too much of your time.

An inherent problem in the way most students solve problems is that they are usually satisfied with the first solution they find. This inherent human characteristic is essential for us to move on with our everyday tasks in a satisfying way without stopping to find the best possible solution for any situation. Although this mechanism might be good enough for most life situations, it could prove insufficient in solving a GMAT exam. Most solvers work their way through a question as fast as they can. When they find an answer that is one of the 5 answers appearing in the question they feel a sense of satisfaction and hence tend to forget to check their answer. The GMAT writers know that and make sure that in every question (where it is possible) a second best answer choice appears. This second best answer choice is the most common mistake made by test takers. Test writers know they only have to fool you in a few questions for you to come to take the test again 31 days later. 
 

Checking your answer in the usual way -going back to the beginning- is a heavy time consumer, and it is usually quite problematic, since most of us are relatively "blind" to our own mistakes (as you probably already know from your own tests experience). Thus, different ways of checking are recommended:

1. Is this what I was looking for? A common mistake is to do everything right and get the right numbers, but not to answer the question you've been asked about. For example: If Dave is twice the age of John and John is 5 years younger than Dave, what is the sum of their ages? When solving the question you get two numbers: 5 and 10. Usually both numbers will appear as possible answers! After reaching the numbers, remind your self what you were asked about -"the sum"- and solve accordingly.
   
2. Is the solution reasonable? Dedicate 2 seconds in order to check whether the answer you've reached is reasonable. For example, if you are asked about the age of a certain person, as in the previous example, reaching the answer 107 for instance, should raise some doubts, and will demand a careful rechecking.
   
3. Plugging the answer you've reached in the first equation used. This step is intended to make sure the answer fits the data given in the question.  

When taking time to review your answers, you give up precious time that might have helped you solve 2-4 additional questions. 
 

Therefore we have 3 recommendations for you:

1. Focus on the first two ways that are mentioned above. These two methods are very quick to implement, and can help you avoid many of your mistakes.
   
2. Take the time to review the first 12-15 questions of each section. Use all three ways mentioned above in order to do so.
   
3. Practice it! Don’t wait until the GMAT exam itself to review your questions! You must be skilful enough with these methods so that the benefit will be greater than the cost (the lost time...).
   Therefore, every simulation that you take must be treated as if it was the real GMAT exam, including the review.