serial control arduino

hi all


hi

how to control variables via serial
variable inside the void do_isr() how i can control them via
serial

#include <TimerOne.h>
volatile int X =1562;

void setup()
{
pinMode (A0, OUTPUT);
digitalWrite (A0, LOW);

Serial.begin(9600);
Timer1.initialize(x);
Timer1.attachInterrupt(do_isr);
}

void do_isr()
{
PINC = 0x01;
for(int i = 0; i <X ; i++)
{

how to change X up or down with serial command for(int i = 0; i <X ; i++)