If you TwoWay bind your TextBoxes to the properties of your business object, they will be automatically updated when the user enters something in the TextBoxes. That's all you need to do. Nothing special.
For more information, please read the topics
under the Row Details section in our online documentation.
In fact, your question could be asked about any
kind of DataTemplate in Silverlight, so it is not particularly relevant to Telerik controls. You can try to do exactly the same thing with the stock Silverlight grid and it will be the same as with RadGridView.
I hope this helps.
All the best,
the Telerik team