WPopt has supported a minimum of version 7.0 of PHP since its inception. In many cases, your site is probably already running on this version of PHP. There can be occasions where PHP 5.6 is the default on your site, but this is easily changed with just a few clicks. In the unlikely event that things "break" you can set things back to as they were before just as easily, no harm done!
1) First, you'll need to log into cPanel. If you're unsure as to how to do this, follow this guide: https://wpopt.net/knowledgebase.php?action=displayarticle&id=3
2) Once logged into cPanel, identify the icon entitled "MultiPHP Manager", under the section "Software"
3) On the next page, simply select your site(s) that you want to change the PHP version for. In the drop-down menu, select the desired PHP version. Note that the PHP version must have a prefix of "ea".
ea-php56 is PHP version 5.6
ea-php70 is PHP version 7.0
ea-php71 is PHP version 7.1
ea-php72 is PHP version 7.2
ea-php73 is PHP version 7.3
ea-php74 is PHP version 7.4
After this, just click "Apply" and you're all done. If your site breaks for any reason, you just need to revert the PHP version to what it was before and no damage will be done!