Larian Studios Forums Divinity: Original Sin 2 - The Modding Dungeon Divinity - Original Sin 2 - Modding General Balancing Skills

Here you go.

https://docs.larian.game/Part_1:_creating_a_new_skill

Click on the part about modifying existing skills at the bottom after reading that page.

Also, this is super helpful:
https://docs.larian.game/Skill_creation