In Canvas, you can create rules for assignment groups, such as a rule to drop the lowest and/or highest scores from grade calculations. To create a rule for an assignment group, navigate to Assignments in the course menu. Click on the Assignment Group Settings drop-down menu and select Edit. From here you can drop assignments with the lowest and/or highest grades. This link describes how to set the rules to suit your preferences: https://community.canvaslms.com/docs/DOC-2626.