You can modify Created Tests and other pre-created Templates within the Create Tests window.
When you Save a Test you have created, it is saved in Test Templates.
Go to the Test Templates screen and select the subject and bank for your test.
Your Tests will be save in the My Templates tab.
Hover over the settings icon for your test and click on Modify.
Your selected test will load in the Create Tests window.
When you modify a Template it does not change the questions within a Published test.
If you need to add extra questions to a Published Test you must first modify the Template and the re-publish it as a new Test.