It is not possible to modify the format or timezone used in the @timestamp field for Beats. It's not a practice we encourage. Why do you want to change the timezone of @timestamp?
This has been discussed in the context of Logstash in another post: How to set @timestamp timezone?
You could probably use a ruby filter to mutate the timezone in Logstash. ES will always store the value as UTC internally. https://www.elastic.co/guide/en/elasticsearch/reference/current/date.html