The posts in your blog are equivalent to the flesh - they give the blog content, features, and personality. The template is equivalent to the skeleton - they give your blog structure. The template isn't visible, like the posts are - but it's equally as important. Change the template, and you change the way all of the posts are displayed.
Hopefully, you back up the entire blog, maybe by mirroring it, periodically. That's a very good idea. You don't want to have to involve Blogger Support, except in extreme situations - like when your blog gets deleted.
(Note): These instructions are specifically written for Classic blogs. They will work for Layouts blogs also - but for Layouts blogs, Blogger provides a GUI template backup for backing up and restoring the template.
It's also a good idea to back up the template. If you make changes to the template, back it up before and after you make changes. Backing up the template is very simple.
- From the dashboard, click on "Template", then "Edit HTML".
- Position the cursor in the Template Editor window.
- Hit Ctrl-A to select all text in the window.
- Hit Ctrl-C to copy the selected text.
- Open Notepad, and position the cursor in the Notepad window.
- Hit Ctrl-V to Paste the copied text.
- Save the copied file.
If you want to modify the template code using Notepad, because Notepad is way easier to use than the tiny template editor window, just make your changes then copy and paste from Notepad back to the template editor window. This will work for Classic or Layouts templates, equally well.
Note: In these procedures, I specify that you use Notepad (or any other plain text editor, that comes with the operating system on your computer) when copying the template. Do not use Microsoft Word, or any other word processor. A simple text processor, like Notepad, is just enough - A word processor, like MS Word, adds control characters. The template is straight text - the control characters, from a word processor, will make your template do strange things.
A template copy takes up very little disk space, and having a backup can be so useful. If you back it up before and after making any changes, you'll have a backup copy in case your current changes don't work as they should. And if the current changes do work as they should, and you need to recover from another calamity, like a mistakenly deleted blog, or a blog hijacking, you'll have a current backup to fall back on.