How do I change the stats of artifacts?

The question is, how can I change the effects of artifacts? For example, how can I modify the "Armor of the Forgotten Hero" so that it provides 10% instead of 20%, or the "King of the Dwarves" set so that it buffs other spells, or the "Phoenix Cloak" so that it provides 50% instead of 30? Thank you in advance for your answer.