Change the horizontal rectangular poster made by PS into a vertical rectangular poster. If you don't consider the deformation of the content, just open the "Image-Image Size", remove the check in the "Constraint Scale" box at the lower left, set the values in the width and height according to your own needs, and confirm.

To keep the original content unchanged, it is easier to redo it than to change it. You can create a new canvas, copy and paste a part of the original content, and then continue to improve it.