Do you mean a model in the UI (like the model of the player in your character sheet), or a model in the world?
1st one:
http://wowprogramming.com/docs/widgets/PlayerModel
2nd one:
You can't, and any model edits are against the TOS.
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"You'd be surprised how many people violate this simple principle every day of their lives and try to fit square pegs into round holes, ignoring the clear reality that Things Are As They Are." -Benjamin Hoff,
The Tao of Pooh