Backing up your template is important, and can one day save you tons of work. Always backup your template before you make any changes to it. This way, if anything goes wrong, you can alway backtrack and everything will be as before.
This is how to backup your template. Sign into Dashboard, select the relevant blog, click TEMPLATE tab and you will be in the Template Edit window. Highlight all the text in the template, right-click and copy (or ctrl+C), open notepad, paste the copied template into notepad. Save with a suitable name. I would suggest the title of the blog followed by the date.