Vanilla

Modifying

To modify a vanilla item, you must get its ItemID. If I wanted to modify the Iron Sword, I would hover it with Advanced Tooltips activated (press F3 + H) and see that its ItemID is "iron_sword".

Then by knowing that all vanilla items have the ModID of "minecraft", I would have the full location of the entry which is "minecraft:iron_sword".

Example:

"minecraft:iron_sword": {
    "attackDamage": 20,
  }

Here I modify the damage attribute and set it as 20.