First upload the favicon image into somewhere that you can access. Then login to your BlogSpot account and go to the Dashboard settings. Then click on Design -> Edit HTML.
Search for ending head html tag.
And paste the following lines before the end tag of head:
<link href='{your image url here}' rel='shortcut icon'/>
<link href='{your image url here}' rel='shortcut icon' type='image/vnd.microsoft.icon'/>
<link href='{your image url here}' rel='icon'/>
<link href='{your image url here}' rel='icon' type='image/vnd.microsoft.icon'/>
Now Save the Template and it will start appear.