You will need to open a HP Support ticket to see if you can change the Discrete GPU inside your laptop. It might be soldered to your Laptop motherboard.
Plus you need to see if that GPU is compatible with your HP Laptop if it is up-gradable. That is why you need to open a HD Support ticket.
NOTE: If for some reason you can't contact HP Support open this Thread at HP Forums. Many times HP Employee are active there and might be able to tell you if it is possible to upgrade the discrete GPU Card in your laptop model.
By the way what is the exact HP Envy X360 laptop model you have. There are like 40-50 models.