// Prepare a collection old and new text
Dictionary<string, string> replacements = new Dictionary<string, string>();
replacements.Add("[[CompanyName]]", "Research Society of XYZ");
replacements.Add("[[EmployeeName]]", "James Bond");
replacements.Add("[[SubjectTitle]]", "The affect of the internet on social behavior in the industrialize world");
replacements.Add("[[TimePeriod]]", DateTime.Now.AddYears(-1).ToString("dd/MMMM/yyyy") + " -- " + DateTime.Now.ToString("dd/MMMM/yyyy"));
replacements.Add("[[SubmissionDate]]", DateTime.Now.AddDays(-7).ToString("dd/MMMM/yyyy"));
replacements.Add("[[AmountReq]]", "$100,000");
replacements.Add("[[DateApproved]]", DateTime.Now.AddDays(1).ToString("dd/MMMM/yyyy"));

// Load diagram
Diagram diagram = new Diagram("FindReplaceText.vsdx");

// Get page by name
Page page = diagram.Pages.GetPage("Page-1");

// Iterate through the shapes of a page
foreach (Shape shape in page.Shapes)
{
    foreach (KeyValuePair<string, string> kvp in replacements)
    {
        foreach (FormatTxt txt in shape.Text.Value)
        {
            Txt tx = txt as Txt;
            if (tx != null && tx.Text.Contains(kvp.Key))
            {
                // Find and replace text of a shape
                tx.Text = tx.Text.Replace(kvp.Key, kvp.Value);
            }
        }
    }
}
// Save the diagram
diagram.Save("FindReplaceText_out.vsdx", SaveFileFormat.Vsdx);