1. Sign into your account by clicking on the "Sign in" link at the top of Justin.tv (if you aren't already signed in)
2. Then click on the "Settings" option on the left-hand navigation menu.
3. Under the "Channel" tab, in the "Info" section, scroll down to the Category and Subcategory drop down.
4. Choose the appropriate Category and Subcategory for your channel.
Remember! The categories aren't changing until May 6th, so the new categories won't appear until then.