Advanced\

  AMD CBS\

    NBIO Common Options\

      GFX Config

        IGC : Forces

        UMA Mode : UMA_Spec

        UMA Frame Buffer : 512 MB

Once you've navigated through the complex page structure of the Advanced menu change IGC to "Forces" (sic. maybe it should read "Forced"). Once you have done that The UMA Mode item appears. Once you have changed that to "UMA_Spec" the UMA Frame Buffer item appears. You can change that to whatever you want but consider this, 2GB is the maximum you can allocate to the iGPU. Auto means that the amount dedicated to the iGPU depends on how much system RAM you have installed and is set at system start up. On my board 16 GB of RAM results in 1 GB being reserved when set to Auto. I understand that 32 GB would result in 2 GB being reserved and 8 GB would result in 512 MB being reserved (so 1/16 of the available RAM, which seems like a reasonable default). However, once RAM is reserved for the iGPU it is not available to Windows so you may not want to allocate the maximum 2 GB, especially since Windows can always dynamically allocate more to the iGPU anyway. Because the iGPU has no special VRAM of its own there is no speed difference between dedicated iGPU memory and dynamically allocated system memory and therefore little to be gained by dedicating a big chunk of the system memory to it unless a specific application (such as a particular game) checks how much GPU memory is present. YouTuber "Hardware Unboxed" did an in depth test of this if you're interested. So please don't simply set the amount to maximum without giving it some thought.