Hey, unfortunately if the option to change your VRAM is not available in the iGPU settings in BIOS, there are very little ways to change the VRAM, especially on a laptop.
A common solution completed by many is using a tool called "Smokeless_UMAF", which lets you access many features hidden by the default BIOS simply by booting from a USB. Although I cannot post a tutorial here, there are many videos online, even for this specific chip.
NOTE: This method is not approved by AMD or BIOS manufacturers so the tool must be used at your own risk.