Rename particular field using logstash 2.4.0?

[Yaswanth]

I have slow logs like this...

[2017-01-13 14:37:48,943][DEBUG][index.search.slowlog.query] [Spectra] [test][4] took[50micros], took_millis[0], types[], stats[], search_type[QUERY_THEN_FETCH], total_shards[5], source[], extra_source[], 
[2017-01-13 12:58:09,844][WARN ][index.search.slowlog.query] [Spectra] [testindex-stats][1] took[16.2ms], took_millis[16], types[], stats[], search_type[QUERY_THEN_FETCH],

I need to rename took[50micros] to took[50ms] in all the slowlogs . How can i able to achieve this using logstash ?

[Yaswanth]

It worked fine with gsub filter..

mutate {
     gsub => [

      "TOOK", "micros", "ms"
    ]
}