Unfortunately RadNumericUpDown does not provide a way to get/set the selection of its internal TextBox. You could try the following code:
var textBox = RadNumericUpDown1.ChildrenOfType<TextBox>().FirstOrDefault();
if (textBox != null)
// manupulate the selection here
but I strongly recommend using it only for testing purposes, since it might add performance penalty on your application.
I will add this request to the product backlog and we will consider it for its future development.
All the best,
the Telerik team