How to clear output buffer of Arduino Uno?
You can't.

The data you are seeing has already been sent by the previous execution of the sketch before the main MCU was reset. That data resides in a small buffer within the USB interface chip (or even within the USB CDC/ACM / USB UART driver itself, depending on your OS) to which you have no access.

You just have to ignore that data. The simplest way is to put a marker at the start of your program to indicate that whatever is receiving the data should start its receiving from this point on. Something like:

Serial.println();
Serial.println("START");