Modding

Hello everyone, how can I add this archer: https://psatkha.narod.ru/creature_433.html, replacing a Tier 2 or Tier 3 unit among the Sylvan elves in Heroes 5.5? He is currently a neutral creature, but I want to replace a Tier 2 or Tier 3 unit in the city. What commands do I need to enter, and how do I change the files, etc.? I'm completely new to modding.