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.

Rules

You can add as many attributes as you want, but you can't repeat them.

There is no set maximum or minimum value, negative values will work, but not for Attack Damage, Flying Speed, and Follow Range attributes.

Movement Speed and Attack Speed attribute are percentages, a value of 1.1 gives the player +110% speed, a value of -0.6 -60% speed and so on.

For Attack Speed a value of -60% equals 1.6 attack speed (average speed). I know its quirky, but it works so I'm letting it be for now.

A value of 0 removes the attribute. To remove Attack Damage, Flying Speed, or Follow Range attributes use -1 or any negative value.