You can edit an existing test via the Teacher environment. When you are here, click on the tests tab. You can select the correct class using a drop-down menu in the top-right of the screen.
Editing a test
You will see a table of all your tests. You can edit your test in three ways.
You can click on the name of your test.
The rightmost column has a few icons you can click on. Click on the pencil icon to edit your test.
You can find your test on the home screen (if it's available and if it's before the deadline of the test has passed). On the bottom-right, you will find a list of the tests you have created and made available to students. You will find the same pencil icon there as well. Click on the pencil icon to edit your test.