Greetings @salmanbpc,
Even if possible, this is highly advised against. See @richgalloway's answer here: https://answers.splunk.com/answers/412085/is-it-possible-to-modify-an-indexed-event.html.
Long story short, you need to delete the index (or clear it of the relevant data), modify the source (log or otherwise), clear the fishbucket, and reindex the data with the proper timestamps. Be very careful doing each of those things if you're in a production environment.
Cheers,
Jacob
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