To change your weigh day you simply have to go through the setup again.
For version 3.7.0 or newer of the app, on the iPhone first go to the More tab, on the iPad this option is located in the left side menu.
Now choose 'Edit User Settings'.
For older versions.
To do this first go to Account Settings (Located in the more tab on iPhone or in the vertical menu on the left on iPad)
Then choose 'Edit User Settings'
You now go through the app setup again allowing you to make changes.