If you log into your Active account using the link below you will have the option to 'transfer to a different category'. You will be able to select the category with the t-shirt and pay the difference in price. https://myevents.active.com/
The following events have a t-shirt included:
Leicester 10K
Sheffield Half Marathon
Derbion Ramathon
Ram 5 Mile
Rob Burrow Leeds Marathon
Yorkshire Marathon
If you wish to edit the size of your t-shirt you can log into your account and edit your entry form up to 6 weeks before the event takes place.