How can change value?

<%
intValue = Request.Form("textbox")
If IsNumeric(intValue) = False THEN
 Response.Write "You entered nonumeric data in fieldname ....blah"
 Response.End
END IF
...
%>

This is a vb example, it might be better to check that with javascript on the form page itself. there are tons of ready scripts out there that validate your form. But double is better as a client might have disabled JS.