JSFiddle - React, Tailwind, and code Playground

HTML

<div id="content">Content Obscured By Overlay
    <button class="deleteMeeting">Delete</button>
</div>

<div id="overlay"></div>

<div id="popupBox">
    <div id="popupContent">
        <i>Are you sure you want to delete this meeting?</i>
        <br />
        <span style="text-align:center;color:black;font-size:40px;">YO</span>
        <br />
        <button class="deleteMeetingButton">Delete</button>
        <button class="deleteMeetingCancel">Cancel</button>
    </div>
    <div id="deleteMeetingConfirmDeleted">Meeting Deleted</div>
    <span class="deleteMeetingClose">&times;</span> <!-- <= added this line -->
</div>

CSS

#overlay {
    display:none;
    opacity:0.5;
    background-color:black;
    position:fixed;
    width:100%;
    height:100%;
    top:0px;
    left:0px;
    z-index: 999;
}
#popupBox {
    display:none;
    position: relative;
    margin-left:auto;
    margin-right:auto;
    margin-top:100px;
    width:600px;
    height: 500px;
    color: #000000;
    border:5px solid #4E93A2;
    -moz-border-radius:8px;
    -webkit-border-radius:8px;
    background-color:#FFFFFF;
    z-index: 1000;
}
#popupContent {
    display:none;
    font-family:Arial, Helvetica, sans-serif;
    color: #4E93A2;
    margin-top:30px;
    margin-left:30px;
    margin-right:30px;
}
.deleteMeetingButton {
    clear:both;
    cursor:pointer;
    width:90px;
    height:30px;
    border-radius: 4px;
    background-color: #5CD2D2;
    border:none;
    text-align:center;
    line-height:10px;
    color:#FFFFFF;
    font-size:11px;
    font-family:Arial, Helvetica, sans-serif;
    font-weight:bold;
}
.deleteMeetingCancel {
    clear:both;
    cursor:pointer;
    width:90px;
    height:30px;
    border-radius: 4px;
    background-color: #5CD2D2;
    border:none;
    text-align:center;
    line-height:10px;
    color:#FFFFFF;
    font-size:11px;
    font-family:Arial, Helvetica, sans-serif;
    font-weight:bold;
    content:"XXXX";
}
#deleteMeetingConfirmDeleted {
    display:none;
}
/* added code below */
.deleteMeetingClose {
    font-size: 1.5em;
    cursor: pointer;
    position: absolute;
    right: 10px;
    top: 5px;
}

JavaScript

$('.deleteMeeting').click(function () {
    $('#overlay').fadeIn('slow');
    $('#popupBox').fadeIn('slow');
    $('#popupContent').fadeIn('slow');
});

// added .deleteMeetingClose into the selectors
$('#overlay, .deleteMeetingCancel, .deleteMeetingClose').click(function () {
    $('#overlay').fadeOut('slow');
    $('#popupBox').fadeOut('slow');
    $('#popupContent').fadeOut('slow');
});

$('.deleteMeetingButton').click(function () {
    $('#popupContent').fadeOut('slow');
    $('#deleteMeetingConfirmDeleted').fadeIn('slow');
    $('#overlay').delay(1300).fadeOut('slow');
    $('#popupBox').delay(1300).fadeOut('slow');
    $('#deleteMeetingConfirmDeleted').fadeOut('slow');
});