Currently there is no easy way to modify the properties of bookmarks and fields in the document. Think of providing API for easier manipulation.

For example, following is one of the easiest approached to change the target of hyperlinks:

                    var hyperlinkStarts = document
                        .EnumerateChildrenOfType<FieldCharacter>()
                        .Where(fc => fc.FieldCharacterType == FieldCharacterType.Start && fc.FieldInfo.Field is Hyperlink);

                    foreach (var fieldCharacter in hyperlinkStarts)
                    {
                        int indexOfCodeRun = fieldCharacter.Paragraph.Inlines.IndexOf(fieldCharacter) + 1;
                        Run hyperlinkCode = (Run)fieldCharacter.Paragraph.Inlines[indexOfCodeRun];

                        string oldUri = ((Hyperlink)fieldCharacter.FieldInfo.Field).Uri;
                        string newUri = "mailto:mail@mail.com";

                        hyperlinkCode.Text = hyperlinkCode.Text.Replace($"\"{oldUri}\"", $"\"{newUri}\"");
                        fieldCharacter.FieldInfo.UpdateField();
                    }