Unfortunately, it's not possible to change the question type in New Quizzes. You have to add the desired question type as a new question and re-enter the prompt/answers.

There is an idea to "Allow Ability to Change Question Type in New Quizzes" that's been added to a theme: https://community.canvaslms.com/t5/Canvas-Ideas/New-Quizzes-Allow-Ability-to-Change-Question-Type-in...