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From the Model Mayhem FAQ:

How do I subscribe or upgrade my account?
You can sign up by heading to our upgrade page here:
www.modelmayhem.com/upgrade

You can also find a link to upgrade on your My Stuff page at the bottom.
Please note that you must already be an approved Model Mayhem member to have a subscription on your account. If you sign up when you register, you will not be charged until your profile is approved.
If you would like to upgrade an existing subscription, simply go to www.modelmayhem.com/upgradeand choose your upgrade and cycle.

Alternately, you can go to My Stuff > Wallet and change your membership type from there.