There is no reason to worry. Free version of the plugin does not require any changes.
When you get pro version of the plugin you will only need to upload it on the site,
install and activate with the key. To do it correctly, please use the instruction in
FAQ section on the main plugin page – https://supsystic.com/plugins/popup-plugin/#faq
So, you see that both versions, FREE and PRO must be installed and activated.