Hi Charles,
I'm afraid the bad news is that the eMMC memory that serves as the hard drive for your laptop is soldered onto the motherboard and is generally not considered to be replaceable.
That being said, there are things you can do to upgrade to Windows 10 if you want, and maximize your existing drive space. There's an excellent post on the HP Community Forums on just that subject that will help you make what space you have much more usable.
[link|https://h30434.www3.hp.com/t5/Notebooks-Knowledge-Base/Updating-Windows-10-OS-on-an-HP-Stream-with-32-GB-eMMC/ta-p/6999369|Updating Windows 10 OS on an HP Stream with 32 GB eMMC memor... - HP Support Community - 6999369|new_window=true]
Hope that helps.