remove css class in code behind

You can replace "required" with an empty string:

lblName.CssClass = lblName.CssClass.Replace("required", "");

This worked for me

lblName.CssClass = "regular";

Just a slightly more generic way of doing the same - should rule out potential errors where a css class might occur elsewhere in the CssClass property.

public void RemoveCssClass(WebControl controlInstance, String css)
{
    controlInstance.CssClass = String.Join(" ", controlInstance.CssClass.Split(' ').Where(x => x != css).ToArray());
}