test.cs

 private static void RenameMergeFields(ApplicationFieldCollection applicationFields,
                                       string oldColumnName,
                                       string newColumnName)
 {
     // iterate through all ApplicationFields
     foreach (ApplicationField applicationField in applicationFields)
     {
         // check if the ApplicationField is a merge field and contains the old column name
         if (applicationField.TypeName == "MERGEFIELD" &&
             applicationField.Name.Contains(oldColumnName))
         {
             // create a new MergeField object and replace the old column name with the new column name
             MergeField mergeField = new MergeField(applicationField);
             mergeField.Name = mergeField.Name.Replace(oldColumnName, newColumnName);
         }
     }
 }