Perform a takeover on the secondary:
hdbnsutil –sr_takeover
When the former primary is available again it can be registered as the new secondary:
hdbnsutil -sr_register
--remoteHost=<new primary hostname>
--remoteInstance=<instance number>
--replicationMode=<sync/syncmem/async>
--operationMode=<delta_datashipping|logreplay>
--name=<siteName>