Arduino uno I2C frequency IDE 1.8.7

I want to change my Arduino I2C frequency from 100khz(default) to 10khz or 400khz which I know the board should be fine with.

But I search for the forum and tried different methods like using Wire.setClock(); after Wire.begin();
or even change the value from 100000L to 10000L or 400000L inside twi.h file then reopen the IDE to upload my sketch. However, none of it worked and it still stays at 100Khz.

I find some topic says I need to delete Wire.o and twi.o which doesn't even exist in my arduino folder.

The code is simple and just for reference so I put it here:

#include <Wire.h>
void setup() {
  // put your setup code here, to run once:
  Wire.begin();
  Wire.setClock(10000L);
  Serial.begin(9600);
}

void loop() {
  // put your main code here, to run repeatedly
  Wire.beginTransmission(10);
  Wire.write(10);     
  Wire.write(5);
  Wire.endTransmission();
  delay(5);
}

Please, help me out.... :sob: :sob:

Why have you put an 'L' at the end of the frequency you want to select?

Well, there are people doing that in some topics and I just followed. However, I tried it with both with 'L' and without, it doesn't work anyway. :confused: