It's in the bar at the top of the schedule screen.
Windows 8 & Windows Phone
It's in the bottom app bar on the schedule screen.
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.
If you do not see this button in the top bar, it is accessible by pressing your device's menu button.
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.