Hi sakshiagarwal311,
Your posts over the last week have all implied that you've just begun your GMAT studies (or are about to sometime soon). The 'content' that is tested on the GMAT has remained fairly consistent over the years, meaning that you would be fine using either of the two Official Guides that you have (and you don't necessarily have to purchase the new GMAT2021 book).
Have you taken any practice CATs/mocks yet? If not, then it would be a good idea to take a FULL-LENGTH practice CAT Test sometime soon; you can access 2 for free from
www.mba.com (and they come with some additional practice materials). If you want to do a bit of practice first, so that you can familiarize yourself with the general content and question types first, then that's fine - but you shouldn't wait too long to take that initial CAT. A FULL CAT takes about 3.5 hours to complete, so make sure that you've set aside enough time to take it in one sitting. Once you have your Score, you should report back here and we can discuss the results and come up with a study plan.
In addition, I'd like to know a bit more about your timeline and your goals:
1) What is your goal score?
2) When are you planning to take the GMAT?
3) When are you planning to apply to Business School and what Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich