Currently, we need 25 luxury resources to upgrade to level 2 star system, 75 to level 3 and 125 to level 4.

I think it is a bit too much and I would like to change it to 25 for level2, 50 for level 3 and 75 to level 4.

However, I could not find the code about resources needed for the system upgrade. Could anyone help me about this?

Hey a3a3311o!

The amount needed for each resource is defined in RecipeIngredientDefinitions.xml.

Each entry corresponds to a resource, and can include regular costs (in normal game, it's one turn per ingredient) and a RecipeCustomCost which has more parameters (such as SlotIndex which is the system upgrade level).