Upon logging in. Go to Settings > Basic > Click Import.
Browse for the backup file ( usuallly an xml file ). Input the captcha word that you see. Once done, click Import Blog.
Importation can help bloggers to transfer their blog post to another one. So, you don't re-type all the blog posts. Or if you having too many sites and you want to handle only weblog you can do this too. Create a back up of all your site then import it to your desired blog site. Take note of the compatibility too. For example, my other site has a back up xml file but it came from a wordpress application, good thing, there are plenty on the web that can change it so it can be applicable to blogger.