You sure can! Head to the "Subscriptions" tab of your user profile and find the subscription you need to update. Click on the three dots at the right of the subscription tab, then select "Change T-shirt size." Your T-shirt size will be updated for all future orders that have not yet begun processing.
If you have a school subscription or a legacy account, you may not have this option in your profile — please submit a request and we'll be happy to assist.