Go to your measurements profile. This is located in the drop down menu which appears when you click your name on the right-hand side of the website’s blue top bar. Click ‘Edit’ to update your measurements and then ‘Save’ once you’ve inputted your changes.
Changes will only be reflected on new orders and orders in the shopping cart. If you need to change measurement details of a shirt which is in production see the ‘About Placing Orders’ section of the FAQ page.