Do you want to redirect all your visitors to the HTTPS version of your blog? For example lets think your blog address is http://pure-gallery-nbt.blogspot.com/. When HTTPS Redirect is turned on, visitors of your blog will always go to https://pure-gallery-nbt.blogspot.com.
Go to “Settings” -> “Basic” and scroll down until you see “HTTPS” section.
Now Just turn on “HTTPS Redirect” setting.
Note: This is not working with third party domains.