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From the Model Mayhem FAQ:

How can I set or reset my custom URL?
You can set your custom URL from My Stuff in Settings. You get one chance to initially set your URL, so make sure you pick something you really like!

The moderation team can reset your URL for you so you can choose another one if you would like. You will need to contact them to reset your URL back to your default ID in order to change your custom URL.