You can edit questions in pools prior to their deployment.
In the Control Panel go to Course Tools > Tests, Surveys and Pools > Pools
On the Pool you wish to edit, click the grey chevron menu and select Edit
Use the grey chevron menu > Edit and update the question or answer you require.
You can still edit questions when users have submitted their Test Attempts, however, this will alter their test performance and may need to be adjusted for in their individual test scores; or the question exempted.