Can I start a topic from the beginning? How?
Yes, you can start a topic from the beginning.
To start a topic from the beginning you have to leave the topic first, in case you are still attending the topic. You can check that in the Topic screen under "My Topics". Here is the article that shows you how to leave a topic.
To attend a certain topic again to start from the beginning, you go to the Topic Screen and scroll down to "More Topics", here you will see the Topics that are available for you. Select the topic you want to do again by clicking the blue "Attend" button.