Set currentDocument property

Best to get an instance of the current document and store it in it's own object, set the values and save the object. Using the CurrentDocument object is not a good idea. Have you looked at the API documentation for working with Documents/Pages?

Here is an example of the right way to get a document and update a property:

private bool GetAndUpdateDocuments()
{
    // Create an instance of the Tree provider first
    TreeProvider tree = new TreeProvider(MembershipContext.AuthenticatedUser);

    // Fill dataset with documents
    DataSet documents = tree.SelectNodes(SiteContext.CurrentSiteName, "/API-Example/%", "en-us", false, "CMS.MenuItem");

    if (!DataHelper.DataSourceIsEmpty(documents))
    {
        // Loop through all documents
        foreach (DataRow documentRow in documents.Tables[0].Rows)
        {
            // Create a new Tree node from the data row
            TreeNode editDocument = TreeNode.New("CMS.MenuItem", documentRow, tree);

            string newName = editDocument.DocumentName.ToLower();

            // Change coupled data
            editDocument.SetValue("MenuItemName", newName);
            // Change document data
            editDocument.DocumentName = newName;

            // Save to database
            editDocument.Update();
        }

        return true;
    }

    return false;
}