Let's say you have the following:
<meta name="author" content="Your Name">
<meta http-equiv="content-type" content="text/html; charset=ISO-8859-1">
<meta http-equiv="content-language" content="en">
<meta name="revisit-after" content="15 Days">
And you want to add <meta name="rating" content="General">
in between
<meta http-equiv="content-language" content="en">
<meta name="revisit-after" content="15 Days">
What you do is search finding
<meta http-equiv="content-language" content="en">
<meta name="revisit-after" content="15 Days">
And replacing it with:
<meta http-equiv="content-language" content="en">
<meta name="rating" content="General">
<meta name="revisit-after" content="15 Days">
This will place that new tag in the correct location.
First make a backup of everything. Then just do it on one page to begin with, no need to test it on the entire website. If it works on the first page, then apply it to the entire web site. View a lot of the pages before you go ahead and publish them.