There is no functional difference between the free and paid version. The free version shows a dialog prompting you to upgrade at a random interval. You can buy a license to remove the dialog.
You can use the free version forever.
Click the upgrade link at the bottom of the purchase page then enter your license information to upgrade.
If you purchased Prepros in the last 6 months you can upgrade for free. You have to pay $19 otherwise. You can continue using Prepros 6 if you don't want to upgrade.
Please follow these links to download Prepros 6.
You can use your license on as many devices as you want as long as you are the only primary user of those devices.
Please enter your email and Prepros version.
Go to Settings app then enable notifications.
Please delete one of the following configuration folders depending on your OS. That will reset all Prepros settings.
Please email email@example.com with your question.