I don't have Y470 but I had other lenovo laptop. If it follows the thinkpad convention, you can find what's "under the hood" by the markings near the bottom cover as they correspond to the hardware component you will find under it. Look for a cylinder shape marking, that is the HDD.
Edit: bad memory, just checked, it should be a rectangle marking. So unscrew the screws with that marking and ply away the cover to find the drive.
Don't really like opening lenovo laptop. They are not screws and lids but more like a 3d lock puzzle!
Make sure power and battery is disconnected, work in well lit hard sturdy surface, keep your screws, ground yourself and press hold power button for like 30 seconds.