How would I remove / replace a HashMap value based off what the Value is?

I have a HashMap that shows a Car Parking space as the Key and a Vehicle Number Plate as the value, I have it so that the user inputs the number plate of the vehicle when they're retreiving their vehicle, I would then like to remove / replace the value for the number plate they just entered.

For example, the output would go from:

Parking Space: 1, Vehicle Registration: ABCD123

to this:

Parking Space: 1, Vehicle Registration: "Empty" / null

the tempVehicle.plate is a String that holds a user entered number plate.

    public int key = 1;
    HashMap<Integer, String> zone_1 = new HashMap<Integer, String>();

    public void addToZone1(Vehicle tempVehicle){
        if (key < 5) {
            zone_1.put(key, tempVehicle.plate);
            key++;
            System.out.println(zone_1.toString());
        }
        else {
            System.out.println("There is no more room in this Zone!");
        }

    }

This is the only thing I have been able to come up with for this, I hope it's possible, I should of looked before hand I guess.

    public void pickingVehicleUp() {
        System.out.println("Please enter number plate: ");
        input = new Scanner(System.in);
        String PLATE = input.nextLine();

        if (zone_1.containsValue(PLATE)) {

        }
    }

I was looking to add onto the if statement or change in completely, whatever I need to do to make this work, thanks in advance