Here's how to do it:
1. Go to your Template page from the Blogger Dashboard. Click on "Backup/Restore".
3. Save the file to a folder that will be easy to find it from again. Also rename to something like "Blog Template" so that it is easy to find.
That's It. If something goes wrong with your template after you make changes, you can bring back your saved template by clicking "Backup/Restore" again (from the Template page). Click "Browse" this time and find the template that you saved and choose "Open".
**One way to avoid ruining your blog when you edit the HTML is to click "Preview" after you make a change and before you click "Save".