To update the subjects that you teach first click the profile tab at the top of the page on your dashboard.

You’ll just need to click the edit button next to the Subjects and Exam boards section:

After selecting 'Add more subjects' you can search for your chosen subjects(s), or scroll through the subject list, sorted alphabetically:

Once you’ve found the subject and subject level you wish to teach, you'll then be prompted to select any exam boards you can teach:

After you've answered these questions, the subject will then be added to your profile!
If you would like some more information about which subjects you are able to teach then please take a look at this FAQ here.