How do I create an irule that rewrites a request to be translated to an ipaddress and port

You should have a pool of servers listening on port 9200, and address and port translation are actually functions of LTM without an iRule. Otherwise, a non-standard port would show up in the incoming Host header, which is pretty straight forward to modify. I'm also adding the URI change code in the example below:

when HTTP_REQUEST {
    if { not ( [HTTP::header Host] ends_with ":9200" ) } {
        HTTP::header replace Host "[HTTP::heeader Host]:9200"
    }
    if { [HTTP::uri] equals "/elasticsearch/" } {
        HTTP::uri "/"
    }
}

This will transparently replace the Host header and URI on incoming HTTP requests. Not sure if you meant to literally only change the URI and Host given a specific URI.