If you have only this card you can try following this video, it is for modifying Samsung BIOS memory of RX 470 normal version but should work in mining edition too, it's the same card after all.
https://www.youtube.com/watch?v=u5Da4KOQFMEIf you have more than 10 cards its best to buy a specific bios mod file from Ebay for 15-20 Euro as it is worth it, but only if you have more than 10 cards available.
Anyway that video should help you quite a lot. Wish you luck in improving your card performance.