Open your existing schedule. Select the date in the calendar view that you would like to remove from the range (e.g. if you want to change the start date and your schedule covers March 20th-May 20th, click on March 20th).
If there are any services on this day, you will need to select and delete them. If there are no services, then click the "New Service..." button and click OK to create an arbitrary service. Once created, select the recently created service and click "Schedule - Delete Service..."
A dialog box will appear asking if you want to change the start date of your schedule to the next day with a scheduled service. When you click OK, this will change the date range of your schedule and it will no longer overlap.