Change all items in the list

using(SPSite mySite = new SPSite(http://mysite))
{
using(SPWeb myWeb = mySite.OpenWeb())
{
SPList contactsList = myWeb.Lists["Contacts"];
foreach(SPListItem existingItem in contactsList.Items)
{
existingItem["E-mail Address"] = "test"
newItem.Update();
}
}
}