how do you update FontAwesome version in mybb

Find out where it's referenced in the HTML(header) and change the ver number in the link.
If it's local you have to download the latest one too.

in this theme there is no version link

{$stylesheets}
<link rel="stylesheet" href="{$mybb->settings['bburl']}/images/majestic/fonts/fa.css">
<link rel="alternate stylesheet" type="text/css" href="" title="red">
<link rel="alternate stylesheet" type="text/css" href="{$mybb->settings['bburl']}/images/majestic/patterns/green.css" title="green">
<link rel="alternate stylesheet" type="text/css" href="{$mybb->settings['bburl']}/images/majestic/patterns/blue.css" title="blue">
<link rel="alternate stylesheet" type="text/css" href="{$mybb->settings['bburl']}/images/majestic/patterns/yellow.css" title="yellow">
<link rel="alternate stylesheet" type="text/css" href="{$mybb->settings['bburl']}/images/majestic/patterns/orange.css" title="orange">
<link rel="alternate stylesheet" type="text/css" href="{$mybb->settings['bburl']}/images/majestic/patterns/navy.css" title="navy">
<link rel="alternate stylesheet" type="text/css" href="{$mybb->settings['bburl']}/images/majestic/patterns/purple.css" title="purple">
<link rel="alternate stylesheet" type="text/css" href="{$mybb->settings['bburl']}/images/majestic/patterns/teal.css" title="teal">
<link rel="alternate stylesheet" type="text/css" href="{$mybb->settings['bburl']}/images/majestic/patterns/brown.css" title="brown">
<link rel="alternate stylesheet" type="text/css" href="{$mybb->settings['bburl']}/images/majestic/patterns/grey.css" title="grey">
<link rel="alternate stylesheet" type="text/css" href="{$mybb->settings['bburl']}/images/majestic/patterns/pink.css" title="pink">
<link rel="alternate stylesheet" type="text/css" href="{$mybb->settings['bburl']}/images/majestic/patterns/cement.css" title="cement">

<link href='//maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css' rel='stylesheet' type='text/css'>

4.7.0 = version