Plesk is a control panel, similar to cPanel. You can change your PHP version inside Plesk, here's a link to the steps showing you how: https://wiki.helionet.org/features/php#change-php-version
Changes do take up to 2 hours to take effect because it needs an Apache server reset. Once you choose the new PHP version you want, you'll get a message banner telling you how long until the new version becomes active.
Next time, since your original request was to add a domain and this thread was marked as [Solved], I'd recommend you make a new thread for a new request topic (like changing your PHP version). It's not a huge deal since I got the notification that you'd quoted me in this thread anyway, but it's more so that further (or new) requests don't get accidentally missed and the helpers here can give you an answer as soon as we're able to. It also helps other users who may be searching the forum for things like how to change their PHP version in Plesk. 🙂
Also, please be aware that WordPress is one of the top reasons that people get their account suspended for High Server Usage. We've made a help page here (https://wiki.helionet.org/misc/wordpress) with suggestions on how to reduce account load. The best suggestion is not to use WordPress in the first place, and there are some great alternatives suggested, but if you cannot move away from WP there are also suggestions on how to try to keep your account load under the shared hosting limits. You can keep an eye on your account load here: https://heliohost.org/dashboard/load/