Edit in JSFiddle

text = "At the heart of GitHub is Git, an open source project started by Linux creator Linus Torvalds. Matthew McCullough, a trainer at GitHub, explains that Git, like other version control systems, manages and stores revisions of projects.";

change = function(word) {
    var r = new RegExp("(" + word + ")", "gi");
    document.querySelector("#content").innerHTML = text.replace(r, "<b>$1</b>");
}

// notes
// text2 = "At the heart of GitHub is Git, an open source project started by Linux creator Linus Torvalds.";
// var element = document.querySelector("#content");
// var changeText = function(word) {
//    var r = new RegExp("(" + word + ")", "gi"); // global match and ignore case flag
//     element.innerHTML = text2.replace(r, '<a href="#">$1</a>');
// }
// changeText('git');
// document.querySelector("#content").innerHTML = document.querySelector("#content").outerHTML.replace(/</g, '&lt;');

<a href="javascript:change('Git');">Use 'Git'</a>
<a href="javascript:change('git');">Use 'git'</a>
<a href="javascript:change('github');">Use 'github'</a>
<a href="javascript:change('pr');">Use 'pr'</a>
<p id="content">At the heart of GitHub is Git, an open source project started by Linux creator Linus Torvalds. Matthew McCullough, a trainer at GitHub, explains that Git, like other version control systems, manages and stores revisions of projects.</p>

a {
    display: inline-block;
    text-decoration: none;
    margin: 0 0 4px 0;
    background: #B0B0B0;
    border-radius: 4px;
    color: #FFF;
    padding: 6px;
}
a:hover {
    color: #000;
}
p {
    margin: 10px 0 0 0;
    padding: 0;
    border-radius: 10px;
}
p b {
    background: #000;
    color: #FFF;
    font-weight: normal;
}