Just sharing a sample code to retrieve the Audit Detail using RetrieveRecordChangeHistoryRequest.
RetrieveRecordChangeHistoryRequest changeRequest = new RetrieveRecordChangeHistoryRequest();
changeRequest.Target = new EntityReference("entitySchemaName", "entityId"));
RetrieveRecordChangeHistoryResponse changeResponse =
(RetrieveRecordChangeHistoryResponse)_service.Execute(changeRequest);
AuditDetailCollection auditDetailCollection = changeResponse.AuditDetailCollection;
foreach...
