if (Gdx.input.isKeyPressed(Input.Keys.TAB)){
    Boolean fullScreen = Gdx.graphics.isFullscreen();
        Graphics.DisplayMode currentMode = Gdx.graphics.getDisplayMode();
        if (fullScreen == true)
            Gdx.graphics.setWindowedMode(currentMode.width, currentMode.height);
        else
            Gdx.graphics.setFullscreenMode(currentMode);
}