All your Questions can be created and edited in your Library (within a Question Set).
Simply hover over the Question you would like to edit, then click on the edit button (the pencil icon) on the right. This will open your Question editor.
Note: Once you start a new Session with a selected Question Set, it creates a 'snapshot' of the Question Set. While you can edit your Questions at any time later in your Library, it will not change the Questions in a previous or currently live Session.