you need to download bios data from your gpu, edit with bios editor like srbpolaris (work only on window) then flahs it back on target gpu.
Be carefull wrong data can compromise your gpu or also impossible to boot it.
If you don't know what to do you need to use window and use amd software to edit freq and voltage to find better then edit your bios with this values.
But what should I edit exactly to reduce power consumption? I am familiar with Polaris bios editing and flashing, but I have never succeeded reducing power consumption.