The Intel HD Graphics Windows 10 driver automatically manages the shared memory for you. By default (if you do not change its configuration), it will support up to half of your physical memory being shared for graphics (presuming that much is ever truly asked for).
If you are truly being limited to 2GB and you did not impose this limit (in the driver's configuration), then this limit is being imposed by your system's BIOS. in this case, if you cannot figure out how to turn this off, you need to contact your board manufacturer for help (we cannot help you; its not our BIOS).
Hope this helps,