Hi pronnoymanish,
To start, a 750 is right around the 99th percentile overall, which means that 99% of Test Takers can't score at that level. Thankfully, no Business Schools actually require a score that high, so the score that you "want" and the score that you "need" are different scores. 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 long have you studied?
2) What materials have you used?
3) How have you scored on each of your CATs (including the Quant and Verbal Scaled Scores)?
Goals:
4) When are you planning to apply to Business School?
5) What Schools are you planning to apply to?
Raising a 510 to a 700+ would likely require at least another 3 months of consistent, guided study, so you might need to consider either pushing back your Test Date or adjusting your overall score goal. Thankfully, the GMAT is a consistent, predictable Exam, so you CAN train to score at a higher level.
GMAT assassins aren't born, they're made,
Rich