This depends on your goals and budget. What schools are you applying to? What GMAT do you realistically expect to get?

I want to apply for mba in finance and was targeting atleast a score of 700. Budget is not a problem because I would be taking sponsorship from my employer company.. is it realistic enough to wait till the last date

Also I have my bachelor's degree in commerce with accounting as my major

If you have under 600 now then applying to schools which require over 700 could be a waste of effort. Why not take a masters in finance? Or take a year to improve your English, which is the way to get a higher GMAT.

so if i apply for masters in finance, wont the GMAT score be a problem there. And what if i write my GMAT again on 7th January and then send my application on 8th January

The problem with your GMAT isn't just the score, but the fact that you're out of balance with a high quant score and a low verbal score. That's less of an issue for some MiFs and more of one for an MBA. At, for example, TU Dallas, Florida State or Fordham a 600 GMAT might be fine for a MiF. At Princeton and Cargegie Mellon, they want a strong score (49, 50) on the quant half of the GMAT, and are less fussy about the verbal side. Indiana University Southeast will accept GMATs over 470 for its MiF!

Of course at the very top schools, Rochester, Berkeley, Iowa, and especially those in financial engineering, the MiF GMAT scores are comparable to MBA GMAT scores or higher.

is it possible edit my scores after i submit the application but before the decision?

Normally schools will allow this but each school may have a different view on how the deal with multiple GMAT scores: some might average them or take the highest score. Ask them.