using DevExpress.XtraRichEdit;
using DevExpress.XtraRichEdit.API.Native;
//...

using (RichEditDocumentServer wordProcessor = new RichEditDocumentServer())
{
    // Access a document.
    Document document = wordProcessor.Document;
    // ...
    // Check all fields in the document.
    for (int i = 0; i < document.Fields.Count; i++)
    {
        // Access a field code.
        string fieldCode = document.GetText(document.Fields[i].CodeRange);
        // Check whether a field code is "DATE".
        if (fieldCode == "DATE")
        {
            // Set the document position to the end of the field code range.
            DocumentPosition position = document.Fields[i].CodeRange.End;
            // Specify a date and time format for the field. 
            document.InsertText(position, @" \@ ""M/d/yyyy HH:mm:ss""");
        }
    }
    // Update all fields in the main document body.
    document.Fields.Update();
}

Imports DevExpress.XtraRichEdit
Imports DevExpress.XtraRichEdit.API.Native
'...

Using wordProcessor As New RichEditDocumentServer()
    ' Access a document.
    Dim document As Document = wordProcessor.Document
    ' ...
    ' Check all fields in the document.
    For i As Integer = 0 To document.Fields.Count - 1
        ' Access a field code.
        Dim fieldCode As String = document.GetText(document.Fields(i).CodeRange)
        ' Check whether a field code is "DATE".
        If fieldCode = "DATE" Then
            ' Set the document position to the end of the field code range.
            Dim position As DocumentPosition = document.Fields(i).CodeRange.End
            ' Specify a date and time format for the field. 
            document.InsertText(position, " \@ ""M/d/yyyy HH:mm:ss""")
        End If
    Next i
    ' Update all fields in the main document body.
    document.Fields.Update()
End Using

winforms-richedit-document-api/CS/RichEditAPISample/CodeExamples/Field.cs#L41

{
    string fieldCode = document.GetText(currentDocument.Fields[i].CodeRange);
    if (fieldCode == "DATE")

wpf-richedit-document-api/CS/DXRichEditControlAPISample/CodeExamples/DocumentFieldActions.cs#L34

{
    string fieldCode = document.GetText(currentDocument.Fields[i].CodeRange);
    if (fieldCode == "DATE")

word-document-api-examples/CS/CodeExamples/FieldActions.cs#L57

// Access a field code.
string fieldCode = document.GetText(document.Fields[i].CodeRange);

winforms-richedit-document-api/VB/RichEditAPISample/CodeExamples/Field.vb#L33

For i As Integer = 0 To currentDocument.Fields.Count - 1
    Dim fieldCode As String = document.GetText(currentDocument.Fields(CInt((i))).CodeRange)
    If Equals(fieldCode, "DATE") Then

wpf-richedit-document-api/VB/DXRichEditControlAPISample/CodeExamples/DocumentFieldActions.vb#L29

For i As Integer = 0 To currentDocument.Fields.Count - 1
    Dim fieldCode As String = document.GetText(currentDocument.Fields(i).CodeRange)
    If fieldCode = "DATE" Then

word-document-api-examples/VB/CodeExamples/FieldActions.vb#L49

' Access a field code.
Dim fieldCode As String = document.GetText(document.Fields(CInt((i))).CodeRange)
' Check whether a field code is "DATE".