Body measurements are not currently editable. Once a set of measurements has been used to create a pattern, they are inextricably linked in our system. Editing the measurements would do some funky, undesirable stuff to your patterns!
I recommend creating a new set of body measurements, using the date in the name so you know which to use going forward! You can pre-fill in your measurements from an existing set by clicking the "copy into new measurement set" button at the bottom of your measurements page.