Try like this -
doc.DocumentElement.RemoveAllAttributes() ;
var xsi = "http://www.w3.org/2001/XMLSchema-instance";
XmlAttribute xmlAtri1;
xmlAtri1 = doc.CreateAttribute("xmlns:xsi");
xmlAtri1.Value = xsi;
doc.FirstChild.FirstChild.Attributes.Append(xmlAtri1);
xmlAtri1 = doc.CreateAttribute("noNamespaceSchemaLocation", xsi);
xmlAtri1.Value = "../xsd/nikuxog_customObjectInstance.xsd";
doc.FirstChild.FirstChild.Attributes.Append(xmlAtri1);
For details check this link -