When you purchase PRO version you should have received an email containing PRO version download link and license key. Download PRO version using that link. You can also download PRO version from Your Purchase History.
Then go to your website Dashboard > Plugins and deactivate Free Version.
Then go to Dashboard > Plugins > Add New > Upload Plugin Then upload PRO version (You just downloaded from email link or Purchase History) then install and activate.
Then go to Dashboard > Front End PM PRO > Settings > License and add your license and “Save Changes”.
Enjoy your PRO features.