On your dashboard, for the blog you want to customize, click the dropdown for more options and select Template.
In the upper right corner, you will see a button that says Backup/Restore. Click that.
Save the template to your computer.
Open it in a text editor and edit to your heart’s content.
Save it as a new template, with a different name. Don’t overwrite your original, just in case you need to restore it back to the way it was. (can’t stress this enough!)
Go back to your Blogger page and use the Backup/Restore feature again, to upload your new template.
I don’t really think you can. The title is represented as data:blog.pageTitle and Google controls what that is, from a combination of the name of your blog (from your blog settings) and the title of your post. If you change that to something else, whatever you replace it with will appear on every page of your site.
So you can’t simply replace it with data:post.title without causing issues on pages that are not post pages, such as the main page of your site, label pages, and archive pages, where multiple posts would appear.
For some things, you do not have as much flexibility with your blog when you use Blogger, as opposed to when using something like Wordpress. There are pros and cons with both, but one of the cons with Blogger is the inflexibility of some of its template tags.