1. Go to "Tools", scroll down to the "Members Booking" section and click on "Casual Booking Rules".
2. Click 
on the right hand side of the Casual Rule you want to change. The contents of the casual rule will populate the table below.
3. There is a section of the table called, "Days Advance Booking:".
4. Change the number in this field and click "Update" at the bottom to save your changes.