HI ankita biswas,
Improving 250+ points in 5 weeks is a difficult goal to accomplish. You mentioned that you needed to submit your scores in October, but your Test Date is in September. How much time is there between your Official Test Date and when you have to submit your scores? I ask because you might be able to push your Test Date back a little bit - and any extra time that you can put towards your studies would help.
It sounds like your plan is to submit for a Round 1 deadline, but could you submit for a later Round instead? I understand that you have a plan that you don't want to change, but you have to remember the REAL goals. If you don't earn a competitive GMAT score, then the application deadlines won't matter. Getting accepted to your first choice Business School is the primary goal and that goal is heavily dependent on your GMAT score.
How much time can you put towards yours studies each week?
Have you considered using any other GMAT resources?
GMAT assassins aren't born, they're made,
Rich