If the new dimensions/measures are either included/not-included in the data source, then the workbook can be opened, data source modified if required, views changed, saved and finally republished using Tableau Desktop.
If the dimensions/measures already exist, then you can edit it using Tableau Server also.
Yes the fields are already existing in the data surce. How can we do it online without going to the worksheet. The users will not have access to the worksheets unless he downloads the workbOok. So how can he do that online?