How to change items in minecraft?

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Change items? You can exchange certain items in creative inventory, or download mods, but I don't know if you can "change" items in minecraft.

If the items are in your hotbar, or the main bar of items on your screen, the slots are associated with numbers on the top of your keyboard, and they are in order from 1 to 9, simply click each number to pick different items.

Depends what system you have.

PC-Scroll with mouse
Playstation: R1 and R2
Xbox: R1 and R2