Check asus website if your model is supported for windows 10.
This doesn't mean anything. G751 was supported since summer, still bug ridden as of october.
You can always downgrade if you don't like it afterwards.
This doesn't always go as well as you would expect.
People here have reported problems that occurred on their systems after upgrade and even more so after a subsequent downgrade.
Also, an ASUS user reported that his "system restore" partition became useless after his upgrade to windows 10
You can upgrade, but you should backup all of your partitions (including EFI partition), if you want to keep the original ASUS system restore partition. I got rid of it and installed windows 8.1 clean from a usb stick.