dropup

bootstrap5.css

bootstrap5.css
.dropup,
.dropright,
.dropdown,
.dropleft {
  position: relative;
}
bootstrap5.css
.dropup .dropdown-menu {
  top: auto;
  bottom: 100%;
  margin-top: 0;
  margin-bottom: 0.125rem;
}
bootstrap5.css
.dropup .dropdown-toggle::after {
  display: inline-block;
  margin-left: 0.255em;
  vertical-align: 0.255em;
  content: "";
  border-top: 0;
  border-right: 0.3em solid transparent;
  border-bottom: 0.3em solid;
  border-left: 0.3em solid transparent;
}
bootstrap5.css
.dropup .dropdown-toggle:empty::after {
  margin-left: 0;
}
bootstrap5.css
.dropdown-toggle-split::after,
.dropup .dropdown-toggle-split::after,
.dropright .dropdown-toggle-split::after {
  margin-left: 0;
}

html bootstrap5 Sample

上側表示のドロップメニュー：dropup

メニュー01 メニュー02 メニュー03 
  <div class="container py-5 d-flex justify-content-center">
    <!-- 上方向へのドロップ -->
    <div class="btn-group dropup">
      <button type="button" class="btn btn-secondary dropdown-toggle" data-toggle="dropdown" aria-haspopup="true"
        aria-expanded="false">上方向へのドロップ</button>
      <div class="dropdown-menu" aria-labelledby="dropdownMenuLink">
        <a class="dropdown-item" href="#">メニュー01</a>
        <a class="dropdown-item" href="#">メニュー02</a>
        <a class="dropdown-item" href="#">メニュー03</a>
      </div>
    </div>
  </div>