Sometimes we need to insert/update item using
“SPSecurity.RunWithElevatedPrivileges(delegate(){});” for getting the Administrator privilege..

But as a result of that we see the “System Account” as a value of “Modified By” field of that inserted/updated item, instead of the original user who actually did the modification.

In this case, we need to update the “Modified By” field in the time of inserting/updating. It may not work if we assign a value for the “Modified By” field and call the Update method.

So we have to use the “UpdateOverwriteVersion()” method. This method allows for the setting of properties on a SPListItem object without creating a separate version of the item. This method also allows for setting of certain system properties such as Created(date), Modified(date), Author(Created By) and Editor(Modifies By).
These are the following code by which we can easily update Modified By field:-

using (SPSite oSite = new SPSite(SPContext.Current.Web.Url))
 {
   using (SPWeb oWeb = oSite.OpenWeb())
   {
      //Variable to store the user
      SPUser oUser = oWeb.CurrentUser;
      oWeb.AllowUnsafeUpdates = true;
 
      SPFieldUserValue userVal = new SPFieldUserValue(oWeb, oUser.ID, oUser.LoginName);
 
      SPList oList = oWeb.Lists["ListName"];
      SPListItem oItem = oList.GetItemById(Item Id);
 
      oItem["Modified By"] = userVal;
                OR
      oItem[SPBuiltInFieldId.Editor] = userVal;
                OR
      oItem[“Editor”] = userVal;
 
      oItem.UpdateOverwriteVersion();
 
      oWeb.AllowUnsafeUpdates = false;
   }
}