JSFiddle - React, Tailwind, and code Playground

HTML

<input type="submit" value="REPLACE INS" onclick="replaceIn()"/>

<div class="post"><em>Post 1: Replace the 'in' with an 'hello'.</em></div><br/>
<div class="post"><em>Post 2: Replace the 'in' with an 'hello'.</em></div><br/>
<div class="post"><em>Post 3: Replace the 'in' with an 'hello'.</em></div><br/>
<div class="post"><em>Post 4: Replace the 'in' with an 'hello'.</em></div><br/>
<div class="post"><em>Post 5: Replace the 'in' with an 'hello'.</em></div><br/>

JavaScript

replaceIn = function(){
    $('.post').each(function(){
	    $('em').html( 
            $('em').html().replace('in','hello')
        );
    });
}