At the Account level. This is the first item under the Advanced section on the Settings page. Setting it here sets the default for the app and will show by default on the Totals tab and in goals.

Associated with a specific Job. You can set it this way in case your jobs have different start of weeks. If they are the same you should use the first option. In order to have the app set the start of week for each Job, you must first set this Job to be seen via the Filter button. The filter button is located on the top right of the screen. The app doesn't apply the Job's start of week as each job could have a different start of week.