After login go to :
https://stackoverflow.com/users/flair
Simply copy and paste the provided HTML snippet wherever you want to show off your Stack Overflow flair. You can find various option (color / combine various SO account ...) to display your flair.
Your link to edit profile:
Update because request in comment:
After complete nerd test its give you html like below:
<a href="http://www.nerdtests.com/ft_nq.php">
<img src="https://www.nerdtests.com/images/ft/nq/5ff9020d2a.gif" alt="I am nerdier than 96% of all people. Are you a nerd? Click here to take the Nerd Test, get geeky images and jokes, and talk on the nerd forum!"></a>
Image url is http
-> change that to https
and use this html on your profile. I use this as a answer.
