Re-downloading Jump Desktop to your device is simple:
Open up the App Store app on your device and re-purchase Jump Desktop from store. Just make sure you use the same iTunes account you used to purchase Jump Desktop earlier. After the purchase is completed you'll be shown an alert telling you that the download is free because you've already purchased the app.
Note: Its very important to use use the same iTunes account you used to purchase Jump Desktop earlier. Otherwise the download will not be free and your credit card will be charged again for the purchase.