How to receive more than 64 bytes on Arduino?
But Arduino has a serial buffer of 64 bytes only. Because of this limitation I am unable to receive further bytes.


This assertion is nonsense. Certainly there is a buffer, but that lets you receive up to 64 bytes (put there by the serial interrupt service routine) before you retrieve a single one.

All you need to do is expeditiously retrieve the data (that is, do a Serial.read()) fast enough that you remove the data from the buffer and store it into memory.

You need to post a Minimal, Complete, and Verifiable Example (MCVE) that demonstrates your problem. For example, are you doing delay(1000) before attempting to read serial data?

See my post about receiving serial data without blocking and also Async (Serial) peripheral interface - for Arduino .