This is a quick and easy (and kinda old but i thought i would post it anyways!) mod for Halo 3: ODST and Halo 3:

1. Extract your profile to your Computer/desktop and then open it up in HxD (or any "Hex Editor")

2. Now, press "F3" or Search for your service tag, Check Unicode String
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3. Now it will show something like this:

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(It will be 3 characters when modding Halo3)

4. Just change both of them to whatever you want... This includes swear words 8-) like this:

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By the way i havent tryed modding the halo 3 service tag.
5. Now just rehash and resign in Modio, and put it back onto your Hard Drive, and Your done!

The End result:
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Remember to +Rep!