Let's say you have a vim session going with a single window/buffer open. If you
were to then open another file with the split command (e.g. :sp README.md),
then you'd have a horizontal split. With one file above and one file below.
You can rotate the orientation of the split to be a vertical split with one
file on the left and another on the right. You can do this with a Window
Command—Ctrl-W H (that's ctrl-w and then capital H).
If you want to go the other direction—from a vertical split to a horizontal
split—you can use Ctrl-W J (that's ctrl-w and then capital J).
This trick only works when there is a split between two windows. When additional splits are involved, it will rotate the focused window to that orientation and leave the others stacked in their current orientation.
See :h CTRL-W for more details on these and other Window Commands.