How to pass a variable inside a function while it is still running

I have a function which reads text file from a sd card. What i want is, while it is still running, somehow i want to change the speed of the charecters read inside the function. How can i do it? It is similarly like when i press a button the music is fast forwarded. Similarly like that, here i want to increase the speed of the text file read by the function. Till now my program is this much:

#include <SPI.h>
#include <SD.h>
#include <stdio.h>
File myFile;
char line[50];
int i = 0;

unsigned long startTime, currentTime;

const int buttonPin = 2;
const int ledPin = 13;
int buttonState = 0;
int count = 0;
int low = 1; 
int t0 = 0; 
bool startTime_on = false;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  startTime = millis();
  pinMode(ledPin,OUTPUT);
  pinMode(buttonPin,INPUT);

  while (!Serial) {
    ; // wait for serial port to connect. Needed for native USB port only
  }

  Serial.print("Initializing SD card...");

  if (!SD.begin(4)) {
    Serial.println("initialization failed!");
    return;
  }
  
}

void file(int n) {
  Serial.print("inside file");
  Serial.println(n);
  String file_n, last_s, first_s;
  first_s = "test";
  last_s = ".txt";
  file_n = String();
  file_n = 1 + first_s + n + last_s;
  myFile = SD.open(file_n);
  if (myFile) {
    Serial.println(file_n);

    // read from the file until there's nothing else in it:
    while (myFile.available()) {
      char c = myFile.read();
      Serial.print(c);
      if(c=='\n') {
        Serial.println();
      }
    }
    // close the file:
    myFile.close();
  } else {
    // if the file didn't open, print an error:
    Serial.println("error opening test.txt");
  }

}

void loop() {
  // put your main code here, to run repeatedly:
  currentTime = millis();
  buttonState = digitalRead(buttonPin);
  
  if(buttonState == HIGH && currentTime-t0 > 100) {
    digitalWrite(ledPin, HIGH);
    if(low == 1) {
      count++; 
      t0 = millis();
      low = 0;
    }
    if(startTime_on == false) {
      startTime = millis();
      startTime_on = true;
    }
  }
  else {
    digitalWrite(ledPin, LOW);
    low = 1;
  }
  
  if(currentTime-startTime > 1500) {
    if(count==1) {
      file(count);
    }
    else if(count==2) {
      file(count);
    }
      else if(count==3) {
        file(count);
      }
      else if(count!=0) {
        Serial.println(count);
      }
    count = 0;
    startTime_on = false;
  }
}

Here file() is the function, in which the text file is being read by the arduino. I’m not getting any idea, how is this possible till now.

Look at the flow of the program. What is executing?

Mark

What i want is, while it is still running, somehow i want to change the speed of the charecters read inside the function.

That does not make sense. Changing when you call the function might. Changing how long it takes to read data does not.