To manage the topics in your study first go to 'collections'. From here you'll be able to add or remove topics to your study using the study hat icon.
If the had is grey then it means this topic isn't in your study. Clicking it will turn it blue and will mean its been added to your study
If the hat is blue it means that this topic is currently in your study, clicking it will turn it grey and remove it from your study.
To see changes to your study you'll need to give the home page a refresh.