Hi,
I have assembled 450 quad with the following hardware.

1. Flight controller KK2.1.5.
2. Lipo battery 11.1 V 3S (orange)
3. ESC 30 A ( Make XXD with 3 A BEC)
4. Propellers: orange 10 X4.5
5. BLDC: Ready to sky 2212
6. Transmitter : Flysky i6
7.Receiver : Flysky iA6.
The issue i am facing is the steps mentioned in KK2.1.5 user manual for calibrating ESC followed correctly, but still M2 is rotating are double the RPM as compared to M1, M3, M4. RPM checked by handheld tachometer. Can anybody help me out with this issueof calibrating ESC.

I hope you've been measuring the motor RPM by removing the props for safety reasons!

Try calibrating the M2 with M2 ESC individually , see if the RPM is within a certain range of error.

Let's say if the RPM of all the 3 motors are 9000 rpm then if the M2 motor is giving 9200 or 8900 rpm it's still okayish.

Also see if your Flight controller is properly leveled!

I am not sure if any of the above methods might work but it's a start! I haven't worked with the board KK 2.1 board since 2015!

Thanks Mohammed will try as u say and will come back.