You can only edit your shirt size for future RT Box orders. To do so please log into My Account through the RT Store, go to My Subscriptions. Under the dropdown for "Manage", select "Swap Product" and update your size there!
If you have already placed your order for your RT Box and need to update the size, please contact our Community Support team here to have them update the shirt size for your current order. Once boxes have begun shipping, you are unable to update your shirt size for the current box.