To manage your registration, you can log in to your profile (via the user icon in the top right corner of the website).
Once you log in you will want to click on your profile icon once more and click on PROFILE
From there you can click "Manage Registration".
To change your T-shirt selection click on "giveaways"
To update your shipping address click on "shipping"
To create and/or join a team click on "teams"
To transfer your registration to a different challenge category click on "transfer challenge" * Note, you will need to re-enter your activity as that data will not transfer.