The lcd screen, touch screen and front glass are already bonded together by strong adheisve, so it is hard to replace only one of them by DIY repair work. Though to buy the front glass replacement is cheaper than the whole screen replacement, but it is more dangerous and you may damage the display or digitizer if you just repair by yourself. And it is better to buy the whole screen replacement. But if you still want to replace the glass only, you'd better ask the professional for help.