by default when the focus comes to that control automatically all the text inside that should get selected with what I have observed some years back in vc6.. not sure of C#..

anyways there should be some property for edit box which makes it EDITABLE so that all the text get selected and user can start typing so that old text gets erased automatically... right click on text box and check the properties referring to option - *EDITABLE ** NON-EDITABLE