Hi bahareh66,
Most Programs accept both the GMAT and GRE as part of the application process, but before you switch Exams, you should make sure that the specific Programs that YOU plan to apply to will accept both Scores. If you are interested in taking a practice GRE Exam, then you access two for free at
www.gre.org.
Before I can offer you the specific advice that you’re looking for, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How many hours do you typically study each week?
2) What study materials have you used so far?
3) On what dates did you take EACH of your CATs/mocks and how did you score on EACH (including the Quant and Verbal Scaled Scores for EACH)?
Goals:
4) What is your overall goal score?
5) When are you planning to apply to Business School and what Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich