Edit the test from the Teacher environment
You can edit an existing test via the Teacher environment. In the side menu, choose Tests.
Find the test you wish to edit, and click on the yellow pencil to edit it.
Change any settings you wish, and don't forget to go all the way to the last step under Settings, in order to click "Save" and confirm your changes.
Edit the test from the home page
It's also possible to make changes to your test from the home page. Just click on the yellow pencil next to your test name.