You edit it in Expression Blend. If you don't have Expression Blend you will need a lot of XAML skills. You will have to write ~300 lines of xaml code I suppose.
I am attaching 2 movies that show how editable is the combobox and how I have created a style fairly close to your screenshot. Also I am attaching the project. It still need some polishing like setting some margins and paddings on some of the elements.
All the best,
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items