All you can really upgrade are the hard drives and RAM, Hwinfo64 will show you exactly what RAM modules you already have be sure to match them to any new ones you want to add.
any 2.5" hard drives/SSD will work Samsung Evo and Pro SSD seem to have the least amount of issues with these notebooks.
Probably Windows 8.1 is the best and trouble free OS, upgrade to 10 at your own risk Asus does not support 10 on these notebooks. Several users are successfully running 10 look through these threads for the drivers they have found to be successful working and prepare your self before doing the upgrade. Have all your win 10 drivers downloaded and ready to install/
Be sure to get an Image of your OS drive before trying to upgrade should you fail you will need this image to recover and get back to where you srarted from
Macrium Reflect is a good Imaging program there are other some work and some don't work with UEFI/GPT disk/SSD. Be sure to create the USB Boot Media and test it to insure it boots. to do this with the USB Media installed boot hitting the Esc key that will bring up a Boot menu select the UEFI Macrium Reflect USB click on it and Macrium should open if it does great if not, go back and re create it again until you get a booting one that works. I have never had a failure but some do and if it won't boot and start the program your image will be usless.
Any other upgrades would require complete disassembly of your notebook.
G752VY-DH72 Win 10 Pro
512 GB M.2 Samsung 960 Pro
1 TB Samsung 850 pro 2.5 format
980m GTX 4 GB
32GB DDR 4 Standard RAM
Z97 PRO WiFi I7 4790K
Windows 10 Pro
Z97 -A
Windows 10 Pro