Easy...

Right click on the calendar name of the calendar that you want to be your default sync calendar and a menu will appear.
Select 'Sync Calendar' from the drop down menu and this calendar will be selected as your default synchronization calendar.



Remember, you can only have one (1) default synchronization calendar. You can quickly visualize what is your default sync calendar because it will have a small green dot next to the sync or don't sync icon as shown below: