<aside> 💡 末尾でjQueryを読み込むこと

</aside>

🔽HTML

<div class="pagetop"><a href=".header"></a></div>

※下の方に設置（bodyの閉じタグか、footerの閉じタグの前くらい）

🔽JavaScript（jQuery）

// トップに戻るボタン
$(document).ready(function () {
  var pagetop = $(".pagetop");
  $(window).scroll(function () {
    if ($(this).scrollTop() > 500) {
      pagetop.fadeIn();
    } else {
      pagetop.fadeOut();
    }
  });
  pagetop.click(function () {
    $("body, html").animate({ scrollTop: 0 }, 500);
    return false;
  });
});

🔽CSS

/* トップに戻るボタンの設定 -------------*/
.pagetop {
  display: none;
  width: 90px;
  height: 90px;
  position: fixed;
  bottom: 30px;
  right: 15px;
  background-color: #962020;
  height: 90px;
  width: 90px;
  border-radius: 50%;
  opacity: 0.7;
}
.pagetop a {
  position: relative;
  display: block;
  width: 90px;
  height: 90px;
  text-decoration: none;
  transition: 0.3s;
  border-radius: 50%;
}
.pagetop a:hover {
  background-color: #fd5757;
  opacity: 0.5;
  border-radius: 50%;
  height: 90px;
  width: 90px;
}
.pagetop a::before {
  font-family: "Font Awesome 5 Free";
  font-weight: 600;
  content: "\\f077";
  font-size: 25px;
  color: #fff;
  position: absolute;
  width: 25px;
  height: 25px;
  top: -60px;
  bottom: 0;
  right: 0;
  left: 0;
  margin: auto;
  text-align: center;
}
.pagetop a::after {
  content: "Page\\Atop";
  line-height: 1.5;
  white-space: pre;
  font-size: 13px;
  color: #fff;
  position: absolute;
  top: 40px;
  bottom: 0;
  right: 0;
  left: 0;
  margin: auto;
  text-align: center;
}
@media screen and (max-width: 767px) {
  .pagetop {
    width: 50px;
    height: 50px;
    bottom: 10px;
    right: 8px;
  }
  .pagetop a {
    width: 50px;
    height: 50px;
  }
  .pagetop a:hover {
    height: 50px;
    width: 50px;
  }
  .pagetop a::before {
    font-size: 14px;
    width: 14px;
    height: 14px;
    top: -40px;
  }
  .pagetop a::after {
    line-height: 1.2;
    font-size: 11px;
    top: 18px;
  }
}