Some weapons and armor in Dragon Age: Inquisition can be modified by way of upgrade slots. In non-crafted weapons and armor, upgrade slots must be pre-existing, they cannot be added by the Inquisitor. Crafted weapons and armor are created with upgrade slots fixed to the equipment. In both cases, the type of potential upgrade slot depends on the type of equipment.
Weapon upgrades themselves can be found in the world, received as a reward for quests and war table operations, or crafted.