You don't..
Chat with our AI personalities
It will cost 800 Microsoft points and you can change by going into "my profile" on the dashboard then select "edit profile" then look for edit/change gamertag. Also you will have to log off and then back on for it to change.
In-Game you are not able to. On the forum you may log on and click "Settings", under "Edit profile" you will see "Edit avatar", click on it and you may change your avatar.
Go to your profile and click edit. After this, click change clan. is not change!is stay on this page!
You have to edit the savegame using action replay, or something similar.
You can only choose the skin you want when you make your sim. After that, you can't change it.