Bootstrap CSS class .dropup

<br> <br> <br> <br>
<div class="btn-group dropup">
  <button type="button" class="btn btn-secondary dropdown-toggle" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
    Dropup
  </button>
  <div class="dropdown-menu">
    <a class="dropdown-item" href="#">Action</a>
    <a class="dropdown-item" href="#">Something else here</a>
  </div>
</div>

/* _button-group.scss:80 */
.dropup &::after,
.dropright &::after {
  margin-left: 0;
}

/* _dropdown.scss:43 */
.dropup {
  .dropdown-menu {
    top: auto;
    bottom: 100%;
    margin-top: 0;
    margin-bottom: $dropdown-spacer;
  }

  .dropdown-toggle {
    @include caret(up);
  }
}