Problem in updating beat.hostname

Hello,

I am facing a problem in updating beat.hostname using mutate filter.I have made the following changes in my logstash configuration file .

input {
beats {
port => "5044"
}
}
filter {
mutate {
update => { "beat.hostname" => "preehana" }
}
}
output {
elasticsearch {
hosts => "localhost:9292"
action => "index"
index => "filebeatl"
}
stdout { codec => rubydebug }
}

You're not using the correct syntax for nested fields. See below.

https://www.elastic.co/guide/en/logstash/current/event-dependent-configuration.html#logstash-config-field-references