I've paid before, how do I get back my previous purchase?

If you downloaded Pro version before, you can still download it without charge from App Store as long as you sign in to the same iTunes Account.

If you purchased the upgrade within the App, you can restore your previous purchase for free. To restore your previous purchase, tap "Upgrade" button and select "Restore". If you delete and re-install the App, or you get a new device, you will need to do restore.