Start reading… its only possible on certain chipset SKUs:
[GUIDE] Coffee Lake CPUs on Skylake and Kaby Lake motherboards - BIOS/UEFI Modding / BIOS Modding Guides and Problems - Win-Raid Forum (level1techs.com)
[GUIDE] Fixing HT for Coffee Lake CPUs on Skylake and Kaby Lake motherboards (Z170, Z270) - BIOS/UEFI Modding / BIOS Modding Guides and Problems - Win-Raid Forum (level1techs.com)
[TOOL] Easy automated Mod tool for Coffee Lake bios - BIOS/UEFI Modding / BIOS Modding Guides and Problems - Win-Raid Forum (level1techs.com)
Any further questions post on the relevant threads linked.