Right now, you can add a measurement you're missing, but already-saved body measurements are not editable.
If you'd like to change one or two measurements, though, you can create a new set by copying from the old one! Click on the name of your measurement set, then scroll down to the bottom. There is a button called "copy into new measurement set" that will pre-populate your new set with your current measurements. You can then make whatever changes you like before saving, and archive the old set once you're done.