Lumen is still working on completing assessment pool editing for Show What you Knows, Self Checks and Final module quizzes in courses with the updated assessment experience (deployed 21 July 2020). As such assessment editing is not yet available, but will be released in September 2020.
If you are on an older course, that still uses the old assessment experience you will have assessment editing available. Please see our Zendesk article for editing quizzes in the old experience.