I hope you are well today and thank you for your question.
You can setup FB Messenger icon for sharing a post in the theme settings as shown in the attached screenshot.
Yes below is the latest version of theme.
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You can update it by either deleting existing theme and then install latest theme version of it or use the following plugin to update the theme.
After activating the above plugin just install the downloaded theme on your site.
You won’t lose your site content after updating the theme but for safer side you can take backup of your site before updating the theme.
What needs to happen when click that messenger share button? It desn’t work here…