How to go into third person minecraft

F5, or on some laptops, Fn + F5 is how you change your perspective between first and third person in Minecraft.

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On some keyboards, for example, my Surface Book, my F5 key both decreases the volume and well, F5.

Depending on the keyboard, Fn makes the keypress do the sub or main function. You may have to experiment with your keyboard, but that key usually means "do the other thing that's also on that key".