First go to Tasks & Worksheets in the Administration view. Go to the 'worksheets' tab. 

To open the worksheet builder press from the worksheet list view or from the detailed view then

Your screen will now include a section on the left where you add/edit Sections and Fields along with the mobile preview to help you see what the user will see whilst you are building your form.

Locate the field that you would like to edit and press followed by

The original view where you initially created the field will open where you can edit any elements of the field.

Further articles are available to describe the Field Types and the Calculations

Don't forget to press when you are done.