When you plan to alter your template, it is always a good idea to save a copy of it to your hard disk drive. The procedure is straightforward.
Sign into your blog and select Layout, then Edit HTML.
The option to Download a copy of your template to disk is displayed on that page.
The file is an XML-file, in order that you know which filetype will be assigned.
Should your alterations wreak havoc on your blog, you can restore the previous status-quo by returning to the same page to upload your original template file. Or a template that you found on the Net; googling for Blogger templates will yield quite a few sites. The usual precautions apply, in terms of security, viruses, malware etc.