In the 'Service Areas' tab, you'll find a section called 'Travel Preferences' by scrolling down past the category tick boxes.
Changing Existing Location Radiuses
To change the radius of a location you have already entered start by clicking the blue 'Show Categories' option in the top right-hand corner of the map. This will bring down a list of all the categories you are listed for and the radius for each one from this particular area.
To alter the radius click the blue link next to the category tick box that lists its current radius in miles. This will bring up a drop-down from which you can select the new distance for each category. The system automatically saves each selection as you update it.