Numeric field needs a default number. It cannot be blank.
As Mohammad Sami told, you can use the tooltip property to display hint.
While entering values into (numeric/text) edit field you can click twice instead of single click. It will select the whole text and when you start typing the old text will be removed.
You an also use tab key to go to the desised field (if that is convenient to you) and in that case also the old values will be preselected and entering new value will replace it.