Unfortunately, the Lenovo product website only shows currently sold models. I could not find the H520. If you go to this page:
http
/shop.lenovo.com/us/en/desktops/#facet-2=4&facet-5=2
After a minute or so an option to chat with someone will pop up. You may want to tell them your exact model and are looking for specifications (CPU's supported, RAM supported, Graphics cards supported, etc.) Maybe they have archived documents somewhere. My guess is you are going to be limited to third generation processors.
Maybe someone else here is more familiar with the H520 desktops and can help out.