First, click the "Grades" link in your course shell Administration block:
Click the "Excel Spreadsheet" link:
Keep everything selected, and click the "Submit" button on the bottom:
Click the "Download" link, and your Excel file containing all of the grades in your course will be downloaded to your computer.
From there, we recommend keeping your course grades spreadsheets in your Google Drive, but that's another blog post…
Again, you REALLY want to make sure you keep records of your students' grades.