Use .flex-column to display the flex items vertically (on top of each other).
Use .flex-column-reverse to reverse the vertical direction, and use .flex-sm|md|lg|xl-column-reverse to achieve this on different screen sizes.
.flex-column-reverse:
.flex-column-reverse
.flex-sm-column-reverse:
.flex-sm-column-reverse
.flex-md-column-reverse:
.flex-md-column-reverse
.flex-lg-column-reverse:
.flex-lg-column-reverse
.flex-xl-column-reverse:
.flex-xl-column-reverse
.flex-xxl-column-reverse:
.flex-xxl-column-reverse