Inventories are a set of Item Collection objects. This makes more sense with an example. Let’s say that a character can have items in their . . .
opsive.com
If you want to modify the existing ItemType amount on an IUsableItem (such as ShootableWeapon) you can get a reference to that ItemAction and then call SetConsumableItemType. This post has an example of getting the ItemAction: https://opsive.com/forum/index.php?threads/inventory.1376/