https://www.elastic.co/guide/en/elasticsearch/reference/6.6/indices-aliases.html has the details, but you want to do something like;
POST /_aliases
{
"actions" : [
{ "remove" : { "index" : ".kibana-$currentone", "alias" : ".kibana" } },
{ "add" : { "index" : ".kibana-$oldone", "alias" : ".kibana" } }
]
}
Just replace $currentone and $oldone in the example above to match the name of the underlying indices.