Your site is running an insecure version of PHP (7.1.33), which should be updated. What is PHP and how does it affect my site? PHP is the programming language used to build and maintain WordPress. Newer versions of PHP are created with increased performance in mind, so you may see a positive effect on your site’s performance. The minimum recommended version of PHP is 7.4. You can test running your site on a different PHP (or WordPress) version using UpdraftClone credits. (Dismiss notice)
OPTION 1: Through back end (DirectAdmin , Cpanel)
Perfect if it needs to apply to all your websites. Not always wishful.
OPTION 2: Through .htaccess
Perfect: if you need different php versions for different websites.
AddType application/x-lsphp73 .php
<FilesMatch "\.(php4|php5|php3|php2|php|phtml)$"> SetHandler application/x-lsphp71 </FilesMatch>