Google Not Updating Last Modified Date on Serps

Guys, how to update my last updated post date on serps? I have tried updating my sitemap and submitting again to google with no luck. I have changed my code for users to view the last updated date and not the published date but google is not updating it, it'll will only show the published date.

I have searched numerous blogs about it and tried them out. But still not working.

You guys can suggest something? Please help. thanks