void print_frequency(unsigned long frequency)
{
float floatfrq;

if (frequency < 1000)
{
  Serial.print(frequency); Serial.println(" Hz");
}
else if (frequency < 1000000)
{
  floatfrq = frequency;
  floatfrq /= 1000;
  Serial.print(floatfrq,3); Serial.println(" kHz");
}
else if (frequency < 1000000000)
{
  floatfrq = frequency;
  floatfrq /= 1000;
  floatfrq /= 1000;
  Serial.print(floatfrq,3); Serial.println(" MHz");
}
else
{
  Serial.print(frequency); Serial.println(" Hz");
}
}



and  the main loop:

void loop() {
 measureFrequency(1000);       // 1000ms standard gate time
     while (_measurement_ready==false); //wait until ready
   print_frequency(_frequency);
}