Can someone explain how to modify psi storm's damage?

Change Upgrade 60 to change the damage of Psi Storm. Note that this also affects various other things (I believe building armor upgrades and worker attack upgrades are affected, to name a few).

In SCMDraft, it's the last "Unlabeled / Unused" upgrade listed under the "Other" category for upgrades.

Psi storm attacks 8 times over the duration of the spell, doing 14 damage each, so if you exit the storm early you take less damage. Adding 1 to the upgrade increases this damage from 14 to 15, so the full damage of the storm is increased by 8 per upgrade. Psi storm ignores unit armor. Damage starts at 112 normally.