Bok Branko,
I need help with my Saphire 2GB RX560.
How can I download BIOS? How can I modify BIOS or even better to ask someone with more experience to do that for me.

You can find how to do basic BIOS mod reading
https://bitcointalk.org/index.php?topic=1954245.0 Or you can buy a BIOS that fit your card from a site mentioned in the guide.
Other option is to buy PBE 2 BIOS modding editor but be advised - it make sence only if you are going to mod many other cards.