Hi emmess26,
First off, a 690/Q45 is a strong score, so it could be enough to get you into your first-choice School. As such, a retest might not be necessary. That having been said, you could pick up a significant number of points in the Quant section, so retesting makes a certain amount of sense. Since you have a specific Program in mind, you would likely find it beneficial to speak with an Admissions Expert about your overall profile. There's a Forum full of those Experts here:
https://gmatclub.com/forum/ask-admissio ... tants-124/With a Q45, your overall 'math' skills are strong, but you likely made some little mistakes in the Quant section on Test Day and you missed out on LOTS of 'strategy-based' questions.
Before I can offer you any additional advice for your studies, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long have you studied? 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)?
4) What is your overall goal score?
GMAT assassins aren't born, they're made,
Rich