You can restart a topic either within the Word Preview list or by tapping on the "Explore" tab.
1. To open the Word Preview list, click on the up arrow (bottom left corner) after a word is introduced. After that, you will be presented with two options: "Continue" and "Reset". Select the "Reset" option to Restart the topic.
2. Alternatively you can tap on the "Explore" tab, select the desired "Category", tap on the desired "Topic" and click on "Reset".
If you want to delete your entire progress, please log into your account, go to Profile, tap on Settings (top right corner), and scroll down until you reach "Reset all Progress".