Click your profile icon on the right side of the app bar. Click View profile.
Next to Teaches, the list of subjects you teach, click on the button that says Manage lessons. To edit an individual subject or lesson package, click the three dots icon next to the subject and then click Edit.
To add a new subject, click the orange button that says Add a new lesson.
When you are finished, click the white button on the top right corner that says Save and exit.