How to Returning Multiple Values From Methods ?

It is not probable to return more than one value from techniques. You cannot, for example, return the licensePlate, speed and maxSpeed fields from a single technique. You could merge them within an object of a few type and return the object. Thus this would be poor object oriented design.

The right way to solve this problem is to describe three separate methods, getSpeed(), getMaxSpeed(), and getLicensePlate(), each of that returns its respective value. For example,
class Car {

String licensePlate = ""; // e.g. "New York 543 A23"
double speed = 0.0; // in kilometers per hour
double maxSpeed = 123.45; // in kilometers per hour

// getter (accessor) methods
String getLicensePlate() {
return this.licensePlate;
}

double getMaxSpeed() {
return this.maxSpeed;
}

double getSpeed() {
return this.speed;
}

// setter method for the license plate property
void setLicensePlate(String licensePlate) {
this.licensePlate = licensePlate;
}

// setter method for the maxSpeed property
void setMaximumSpeed(double maxSpeed) {
if (maxSpeed > 0) this.maxSpeed = maxSpeed;
else this.maxSpeed = 0.0;
}

// accelerate to maximum speed
// put the pedal to the metal
void floorIt() {
this.speed = this.maxSpeed;
}

void accelerate(double deltaV) {

this.speed = this.speed + deltaV;
if (this.speed > this.maxSpeed) {
this.speed = this.maxSpeed;
}
if (this.speed < 0.0) {
this.speed = 0.0;
}
}
}