Please see the Order by property of the Flexible block: Flexible block
Be careful if doing this for an EIT, SIT, or any other list which supports pending transactions because the segment or attribute value will be null or outdated and you may have to consider _p values. For example:
&pOrderBy=to_date(nvl(segment1_p, '0001/01/01 00:00:00'), 'YYYY/MM/DD HH24:MI:SS') desc, to_date(nvl(segment1, '0001/01/01 00:00:00'), 'YYYY/MM/DD HH24:MI:SS') desc
Please refer to: Data objects