To update the subject 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 subject section:
All the subjects are organised into categories, use the right and left arrows to scroll through the categories:
Once you’ve found the subject you’re looking for simply click on the subject and/or level you wish to add.
The subject will be highlighted in dark blue if it has been selected and gray if it has not.
If you would like some more information about which subjects you are able to teach then please take a look at this FAQ here.