You need to get the input element then to get or set the text :
HtmlElementCollection elements = currentBrowser.Document.GetElementsByTagName("INPUT");
foreach (HtmlElement element in elements)
{
//to get the text use : string value = element.GetAttribute("value");
//to set the text use : elemet.InnerText = "something";
}
but don't forget that by the code above you get all the input elements , to search for a specific element you can check its id or name as in the webpage : for example :
if (element.Name.ToLower().Contains("email"))
//do work