Go to the object editor, and click on the upgrade tag.
I reccomend, looking through the upgrades and picking the one that best describes what your looking for. Probobly a damage bonus upgrade.
Create a new upgrade based off that. You can now change the initial cost, and the cost-per-level factors. You can also change the icons and names for each level.
You may have to watch for the class and type of the upgrade. Some might say Melee units, and others ranged, so you might have to change that too.
Then, go to the units tab. In all of the units you want effected by it, add your new upgrade to 'Upgrades used' field.