This is done as part of Moderating Quizzes; note that you must have published a quiz before you can moderate it. If you have set a time limit on your quiz, you can grant access for extra time. If the student hasn't taken the quiz, the extra time will be added to the student's initial attempt and additional attempts. If a student is currently taking the quiz while you are viewing the Moderate Quiz page, you can extend the quiz time as part of their current attempt. You can choose how many minutes to extend the quiz, and whether you want to add those minutes to the current time or the current (scheduled) end time.
Once I publish a timed quiz, how can I give my students extra time? (Canvas Community)
When you create a page in Canvas, anything that you input in the Rich Content Editor is checked by the Accessibility Checker.
How do I use the Accessibility Checker in the Rich Content Editor as an Instructor? (Instructure Community)