Unfortunately, I don't have a prescription for how to improve a score by 90+ points in 12 days. Most people would need a couple of months at minimum. In general, what you're scoring 7 to 10 days before the real test is what you'll probably score on the real test.
What is considered a "good" score varies. It depends on the programs to which you're planning to apply. If you haven't done that kind of research yet, then you actually need to start there. That will help you to set your goal score.
So my best advice for you is to postpone your exam, sit down and thoroughly analyze your study approach, strengths, and weaknesses, and come up with a realistic plan to achieve a 650+ score (or whatever score you decide is your goal after researching the programs to which you'd like to apply).
First, read these two articles:
http://tinyurl.com/executivereasoninghttp://tinyurl.com/2ndlevelofgmatThink about how what you've been doing does and doesn't match up with that and how you may need to change your approach accordingly.
Then, use the below to analyze** your most recent MGMAT CATs (this should take you a minimum of 1 hour):
http://tinyurl.com/analyzeyourcatsBased on all of that, figure out your strengths and weaknesses as well as any ideas you have for what you think you should do. Then come back here and tell us; we'll tell you whether we agree and advise you further. You may also want to look at this article on developing a study plan:
//www.manhattanprep.com/gmat/blog/2014/01/15/new-years-resolution-get-your-score/
** Note: do share an analysis with us, not just the raw data. Your analysis should include a discussion of your buckets - you'll understand what that means when you read the Analyze Your CATs article. Part of getting better is developing your ability to analyze your results - figure out what they mean and what you think you should do about them!