How do I restart opensearch ?

AFAIK, there is'nt a way to restart AWS Opensearch cluster or nodes individually because that's mostly handled by AWS if they find some issue with the underlying nodes. But if the domain endpoint is responding to API calls you can try to identify why the cluster is red, which indices and shards are causing the issue and further identify the next course of action. Depending on the diagnosis you might also need to add data nodes. This document might be useful : AWS KB

--Syd

Now, Amazon OpenSearch Service offers several administrative options which include the ability to restart the OpenSearch process on a data node and the ability to restart a data node. [+] https://docs.aws.amazon.com/opensearch-service/latest/developerguide/admin-options.html