I set up 60 total points for a test in Canvas, but 10 are supposed to be extra points. If a student earns 60 points, 10 should be his/her bonus points. How should I make this adjustment after all the students took the test?
If you are using Classic Quizzes the total points of the quiz will always be the sum of the points for each question. This makes it very difficult to change and do what you are asking. My suggestion in this case would be to create a new assignment set for no submission with a value of 50 points. Then enter their grade into that assignment. For the original quiz, edit it and change from Graded Quiz to Practice quiz. This will keep it it in the course for students and you to review, but will remove it from the gradebook.
If you are using New quizzes you can change the total point value for the quiz independently of the sum of the points for each question. However, it scales the grade when you do. So for your example if a student has 60/60 right now, when you change it to 50 I believe their grade will scale to 50/50. What you could do in this case is to make that change, and then in Speedgrader give everyone 10 more points in the Fudge points section, or simply record their score before the change then enter it after you've made the adjustment.