If your talking about Modern Warfare 2, go to callsigns/killstreaks, and click Emblem.
If your talking about Black Ops, go to player card, edit player card.
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2nd prestige
Ten Tactical Nukes to unlock the emblem.
It happens to be a title, not an emblem. You're supposed to get 25 kills with fragmentation grenades.
You unlock it by getting to 10th Prestige.