To download grades from a Canvas course, first select "Grades" from the left navigation pane. Then under "Actions," choose "Export."

This will immediately begin a file download that you can open in Excel. Mine appears at the bottom of the screen because I'm in Chrome. Depending on how many grades you have, it may take a while. If you try to export grades from a course you previously exported from, you'll notice past exports in the dropdown. You can run a current export by clicking on "Export" like you did the first time, or you can access previous exports by clicking on them.