It’s usually recommended that these types of snippets be placed in your theme’s functions.php file, but I personally don’t think that’s a good idea since depending on how the theme update works you may lose these custom changes when updating your theme. Some people use the Functionality plugin or something similar for this purpose.
If you choose to place this snippet in your theme’s functions.php file please proceed with extreme caution. Make a backup of the file before editing it and ensure that you have access to your site’s file system via FTP or your hosting account’s cpanel just in case something goes wrong.