In most cases, the motherboard manufacturer has already configured the system to use the maximum amount of graphics memory that is possible for your particular computer model. Sometimes you can change these values through BIOS level.
Below is a URL that explains what is the maximum amount of graphics memory your graphics controller could use:
Check with your motherboard manufacturer if there are tips on increasing the amount of graphics memory for your computer model.
Oh my God! I'm having the same problem with ASRock B150M Pro4S motherboard! This is nightmarish! It's my first build ever and I won't buy ASRock product ever! They are happily presenting themselves as the only major motherboard manufacturer featuring 1792GB of shared memory yet my BIOS (fully up-to-date) is locked at 1024MB.
Damn! I even bought a 16GB DDR4 kit for this!