On all of our apps, you can add, edit and delete subjects from the Manage Subjects screen. How you get to this screen depends on the platform you're using.
The web
It's in the bar at the top of the schedule screen.
Android
It's accessible from the schedule screen however how you find the option will depend on the device and version of Android you're using. If you can see a button in the top bar which looks like this then tapping it will show multiple options, one of which is Manage Subjects.
iPhone
In the top bar on the schedule screen there is an ellipsis button (3 circles side by side). Like the Android app, tapping it will show multiple options, one of which is Manage Subjects.