How do I retrain google net's classification to suit my project?

Hi, I am currently doing a project on object recognition and I am using Google Net to be my preferred CNN. However, some of the classification such as sports car, garbage truck is too specific. I would want to make it more general such as to classify it as a car and just a truck. Here is my code, help would be appreciated :

clear

camera = webcam; % Connect to the camera net = googlenet; % Load the neural net net.Layers

while true picture = camera.snapshot; % Take a picture sz = net.Layers(1).InputSize ; picture = picture(1:sz(1),1:sz(2),1:sz(3));% Resize the picture

    label = classify(net, picture);        % Classify the picture

    image(picture);     % Show the picture
    title(char(label)); % Show the label
    drawnow;   
end