There are many different ways to give extra credit to students in Canvas.
Listed below are several popular options with direct links to picture instructions on the Canvas Instructor Guides.
How do I give extra credit in a course?
Add extra points assignment to a specific “Extra Credit” Assignment Group
Add extra points using a zero point assignment with “No Submission”
Bonus! Workaround on how to make an “Extra Credit Quiz” in Canvas
Making an entire quiz worth extra credit isn't easy in Canvas if you are using a Points based gradebook. The following directions explain how to make this happen:
Create a quiz
Provide point values for each question. (making the quiz worth "X" amount of points - the total number of points the extra credit should be worth.)
Publish the quiz and have the students take it.
Once all students have taken a quiz and received a score, which is recorded in the gradebook.
Edit the Quiz and change the Quiz type to a Graded Survey worth "0" points.
The student will then receive whatever extra credit value they earned.
IMPORTANT: When copying the course it will copy it as a survey - so be sure to Edit and revert it back to a graded quiz for the next term!!