If you need to add a subject to an existing Simple Assessment scheme, you can do this by first following this guidance: How can I add a Subject that isn't listed?
Once you've added the subject, you can include it to the existing scheme via Admin > Config > Simple Assessment Config and clicking Edit next to the scheme you wish to change.
The existing subject will be greyed out (you cannot remove it once data has been entered).
You can select a new subject to add by expanding the subject categories and ticking the subject you want which will show in blue.
When you've finished, scroll to the bottom of the page and click 'Update Assessment'.