You can choose the order in which your categories and services appear in online booking. This can be different for each location if you have more than one.
Open local admin settings and do the following:
Click admin in the right hand menu
Choose your location in the drop down under the page title at the top of the screen
Click Services in the box on the left
Click on the Category or Service you wish to choose the order of
Click the "Online Booking" section
The first field is "Display order" - enter the number that you'd like the Category or Service to appear in (1 being at the top)
Click Save
Ensure that you do not have two Category or Services services saved with the same number.