Modify upgrade effect

[RobertMKD]

How do I modify the effect an upgrade? Like increase the amount of armor 'Steel Armor' gives..

 

[DEE-BOO]

To upgrade armor, I think you have to modify each unit individually under Data - Defense Upgrade Bonus. So any armor upgrade will increase that unit's armor by that amount.

 

[Wietlol]

Most upgrades simply are set as requirements to unit/building creation or have their effect set in the data of the upgrade itself (it then applies on all units that have that upgrade in used upgrades).

However what I understand from your question is how to increase the effectiveness of a certain armor type.
This can be done in the Gameplay Constants where you can find a table of real values which are factors of the damage taken from the attack type to the armor type.