Step 1:-


Go To Blogger.com >> Your Blog >> Theme>>3 dots>>Backup Theme.
Now Backup your template.

Step 2:-


The First thing you need to do is log in to your Blogger account and go to >> Templates >> Edit HTML and search for the ending </body>.

Step 3:-


Now just before or above you have to paste this below code. 

<style>
#sbtbacktotop{ background: url(https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgNPVu-eD86-BNOyvhsA0RyFMbJuMzZTTJqcyq3mgvgGOFFO7suQUO4B9_ontgEMWxN4_kAnA_U6Fa0JAe25zFhN1wBuZFr8pTEacasxvLXLKJTGbw2Sp5Zo6F-eN4m393H-2JpJ4ChyL5Q/s1600/back-to-top-sprite-30224d9b.png) 0 0 no-repeat;
height: 130px; width: 72px; padding:5px; position:fixed; bottom: 5px; right: 5px; cursor:pointer; transition:none;     z-index:15; }
  #sbtbacktotop:hover{
    background:url(https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgNPVu-eD86-BNOyvhsA0RyFMbJuMzZTTJqcyq3mgvgGOFFO7suQUO4B9_ontgEMWxN4_kAnA_U6Fa0JAe25zFhN1wBuZFr8pTEacasxvLXLKJTGbw2Sp5Zo6F-eN4m393H-2JpJ4ChyL5Q/s1600/back-to-top-sprite-30224d9b.png)no-repeat;
background-position: 0 -142px; } </style> <!-- Back to top button by SBT --> <script src='http://ajax.googleapis.com/ajax/libs/jquery/1.10.0/jquery.min.js'/> <script type='text/javascript'> //<![CDATA[ $(window).scroll(function(){if($(this).scrollTop()>100) {$("#sbtbacktotop").removeAttr("href");$("#sbtbacktotop").stop().animate( {bottom:"0"},{duration:100,queue:false})} else{$("#sbtbacktotop").stop().animate({bottom:"30000"}, {duration:8000,queue:false})}});$(function() {$("#sbtbacktotop").click(function() {$("html, body").animate({scrollTop:0},"slow"); return false})}); //]]> </script> <!-- Code provided to you by sbt --> <a href='#' id='sbtbacktotop'></a>