Hey Sachin! If it's of any consolation, I've done the same thing. As an international student I wasn't sure about sixth form systems etc during my first cycle of apps, and I actually submitted my O level Grades to HSF under the 16-18 studies portion (where I was actually supposed to key in my A level grades...). Looking back, I now know A level grades are required by almost every firm, but there I was submitting an app without my A level grades!
Anyways I emailed the graduate recruitment team explaining that I misunderstood the grades portion of their application, submitted the correct grades for them to factor in and they took it well and amended my app accordingly! Got the offer for their spring scheme in the end, so don't worry about it diminishing the quality of your application! Mistakes like these happen to the best of us, and we can't be expected to know everything! In fact owning up to your mistake and emailing them promptly demonstrates you taking initiative
The email I sent if it helps:
Dear Graduate Recruitment Team,
My name is ... and my candidate number is ... Upon reviewing my application, I've noticed that my grades for my 14-16 studies were put under the 16-18 section. I have submitted my A-level grades as part of a separate document attached to my application (this was completed when I was 18, but I enrolled into the school when I was 17 owing to the different education system in [my home country]), and the correct 14-16 school examination grades are as follows: English- X, Science (Chemistry and Biology)- X, Mathematics- X. I apologise for this misunderstanding and I hope this information aids in any confusion! Thank you for your time in reading this.
Best regards,
[My name]