You can change the business hours, by logging on and clicking on your business name at the top of the page, then selecting "More Options" at the bottom of that window, then choosing the "Business Hours" Tab

You can also access the same window via "Admin|Settings|Company Details" in the menu system in the top left

You may also have to refresh the browser view in order to apply the changes to the current view. You can do this by pressing the F5 key (or the little refresh icon, which looks like a little arrow going around in a circle, near the browser URL bar)

Furthermore you may also need to change the employee hours to reflect the new business hours!

You can find how to change the employee hours here:
http://diarybooker.uservoice.com/knowledgebase/articles/117063-how-can-i-change-the-employee-hours