JSFiddle - React, Tailwind, and code Playground

HTML

<div id="replace-me">Hello, world</div>

JavaScript

replaceWordAtIndex = function (text, index, replacement) { 
        text = text.split(' ');
        text[index] = replacement;
        text = text.join(' ');
        return text;
    }
    wrapWordAtIndex = function (text, index, before, after) { 
        text = text.split(' ');
        text[index] = before + text[index] + after;
        text = text.join(' ');
        return text;
    }
    
    var obj = document.getElementById('replace-me');
    obj.innerText = replaceWordAtIndex(obj.innerText, 1, 'Universe');
    obj.innerHTML = wrapWordAtIndex(obj.innerText, 1, '<b>', '</b>');