Change the value of the private member in the class designed by singleton pattern

Today I have a question about this code.

class Engine
{
    static Engine* getInstance()
    {
        return instance = (instance == NULL) ? new Engine() : instance;
    }
    SDL_Renderer* getRenderer() {
        return mRenderer;
    }
    bool init();
    bool Clean();
    bool Quit();

    void Update();
    void Render();
    void Events();

    SDL_Renderer* getRenderer()
    {
        return mRenderer;
    }
private:
    Engine();
    static Engine* instance;
    SDL_Renderer *mRenderer;
};

Engine* Engine::instance = NULL;

If I want to change the value of mRenderer as like a reference variable, I must write this code void render(Engine::GetInstance()->getRenderer()). So I don't know how the mRenderer change are.

Thanks for reading.