UPDATE: If you are using the new Blogger dashboard, read : "How to Back up your template and widgets here".
If you are a Blogger user, it is always advisable that you Back Up your template before making any changes to "EDIT HTML" under the template tab. The main reason for this is to make it easy for you to restore the template should in case the changes you wish to effect aint successful thereafter. There are two simple ways to do this in the new beta blogger.
The first method is to click the 'Download Full Template' link under the heading Backup/Restore Template on the ''EDIT HTML" page. Click on it and save the file to your hard disk.
The second method is to first put a check in the box to 'Expand Widget Templates'. This reloads the page and expands the template in the html box. Now right click anywhere within that text box and choose 'Select All' Then right click within the blue selected area and choose 'Copy'. Open Notepad, Right click within it and choose 'Paste'. This pastes your entire template in Notepad and now save it as a text file on your pc.
It is advisable that you save it with a name that reflects the day, even the time you created the Back Up so you can always know the most recent out of the template Back Ups you might have on your pc. You can revert to your saved template by simply Uploading it with the ''UPLOAD'' button under the 'Download Full Template' link on the 'EDIT HTML' page.
I hope this short info helps..