When an AI is holding a weapon such as the included BP_AISword weapon the AIs damage is controlled by the Damage setting found in the BP_AISword blueprints Class Default, here you can control the damage values for each hitbox type.

When the AI is not holding a weapon its damage value are controlled in its BP_AIComponent. To find these go to the AI characters blueprint like for example the BP_Zombie and select the BP_AIComponent in the Components panel, then in the Details panel search for Damage to find the settings.

Here you can set both the melee and range damage values for each hitbox the AI uses when not holding a weapon.