For In-App Purchases (Available Since Version 3):
This should be fully automatic, as long as you download and install the app using the same Google or Amazon account that you originally purchased under. Once installed, the app should be immediately unlocked for unlimited use. In the unlikely event that this does not occur, take a look at this article for troubleshooting tips.
For Google Play and Amazon Purchases Made Prior to Version 3 (December 2014):
Prior to version 3, a separate license "app" was purchased from Google Play or Amazon. If your device is linked to the same Google/Amazon account as before, you should be able to download the license again at no charge.
In Google Play, go to the “My Apps” area to begin the download. On an Amazon Kindle, go to the apps area and tap on the "Cloud" section.
If you don’t see the app listed, try searching for “Ultimate To-do List License” or “Ultimate ToDo List Tab License” (depending on your device).
If you are asked to pay again, this means that you are using the wrong Google/Amazon account. In Google Play's settings, try switching to a different Google account. Or, go to your device’s settings area and set up a link to the correct Google account. Note that you can have a device linked to more than one Google account at the same time. On a Kindle, you will need to go to the settings area and link the device to a different Amazon account.
For PayPal Purchases Made Prior to Version 3 (December 2014):
Send an e-mail to firstname.lastname@example.org and we will send you instructions on how to unlock the app. Please provide enough information for us to locate your purchase, including your registration code and the e-mail address the purchase was made under. The registration code is located in the email you received at the time of purchase.